Science

100 Best Science Facts That Explain How the World Works

100 well-established science facts about atoms, chemistry, life, the human body, Earth, weather and space, each with a short explanation of why it is true.

By BlogAI Editors · · 26 min read

Most "fun fact" lists give you a surprising sentence and stop there. This one tries to go a step further: each of these 100 science facts comes with a short explanation of why it is true and what it tells you about how the world works. Read together, they add up to a rough map of modern science, from atoms to galaxies.

The facts are grouped by topic: matter and atoms, chemistry, energy and forces, life, the human body, Earth, water and weather, and space. Numbers are rounded and come from textbooks, scientific agencies and peer-reviewed papers, which are linked in each section and listed under References.

About this list. Everything here is mainstream, well-supported science, not speculation. Where a figure is an estimate (the number of cells in your body, the temperature of Earth's core), we say so. For physics in more depth, see our companion list of 100 physics concepts explained with examples.

Matter and atoms

Everything you can touch is made of atoms, and a surprising amount of everyday behaviour follows from how atoms are built (OpenStax Chemistry 2e, chapter 2).

1. Atoms are mostly empty space

An atom's nucleus holds almost all of its mass, yet it is roughly 100,000 times smaller across than the atom itself. If a nucleus were the size of a marble, the atom would be about the size of a sports stadium. The rest is occupied by electrons, which behave less like tiny planets and more like clouds of probability.

2. The number of protons decides the element

Every carbon atom has 6 protons, every oxygen atom has 8, and every gold atom has 79. Change the number of protons and you have a different element. This number, the atomic number, is what orders the periodic table.

3. The same element can have different weights

Atoms of one element can carry different numbers of neutrons. These versions are called isotopes. Carbon-12 and carbon-14 behave almost identically in chemistry, but carbon-14 is radioactive, which is what makes radiocarbon dating possible.

4. Chemistry is mostly about electrons

When atoms react, their nuclei stay the same; only the outer electrons are shared, swapped or rearranged. That is why chemistry cannot turn lead into gold, but nuclear reactions, which change the nucleus, can turn one element into another.

5. There are 118 known elements

The International Union of Pure and Applied Chemistry (IUPAC) recognises 118 elements, the most recent four named in 2016 (IUPAC). About 90 occur naturally on Earth; the heaviest are made in laboratories and last only fractions of a second.

6. Hydrogen is the most common element in the universe

By number of atoms, roughly 9 in 10 atoms in the universe are hydrogen, and most of the rest are helium. Both were made in the first few minutes after the Big Bang. Nearly everything else, including the carbon in you, was made later inside stars.

7. You are made of star material

Elements heavier than helium, such as carbon, oxygen, calcium and iron, were forged by nuclear fusion in stars and scattered when those stars died. The calcium in your bones and the iron in your blood were made in stars that existed before the Sun (NASA).

8. Solid, liquid and gas are about how tightly particles are held

In a solid, particles vibrate in fixed positions. In a liquid, they stay close but slide past each other. In a gas, they fly freely and fill their container. Heating a substance gives its particles more energy until they break free of the forces holding them in place.

9. Most of the visible universe is plasma

Heat a gas enough and electrons are stripped from atoms, creating plasma, a soup of charged particles. Stars are made of plasma, so by mass most ordinary visible matter in the universe is in this state, even though it is rare in daily life on Earth outside lightning, neon signs and flames.

10. Glass is a solid without a crystal pattern

Glass is an amorphous solid: its atoms are locked in place like a solid but arranged randomly, like a frozen liquid. The old story that medieval windows are thicker at the bottom because glass flows is a myth; the panes were simply made unevenly, and glass does not flow measurably at room temperature.

11. Diamond and graphite are the same element

Diamond and pencil graphite are both pure carbon. In diamond each carbon atom bonds to four neighbours in a rigid 3D network, making it extremely hard. In graphite the atoms form flat sheets that slide over each other, which is why graphite leaves marks on paper.

12. Mass is conserved in chemical reactions

When wood burns, it seems to disappear, but the atoms are not destroyed. They leave as carbon dioxide and water vapour, and the total mass of everything before and after the reaction is the same. Antoine Lavoisier established this principle in the late 1700s.

Chemistry in the everyday world

Reactions explain why food browns, metal rusts and soap cleans (OpenStax Chemistry 2e; American Chemical Society).

13. Rust is slow burning

Rusting and burning are both oxidation: a substance combining with oxygen. Burning is fast and releases heat and light. Rusting is slow and needs water as well as oxygen, which is why iron rusts faster in damp, salty air.

14. Fire needs three things

A fire needs fuel, oxygen and heat, the "fire triangle". Remove any one and it goes out. Water cools, a fire blanket cuts off oxygen, and a firebreak removes fuel. This is also why you must never throw water on a burning oil pan: the water flashes into steam and throws burning oil out of the pan.

15. The pH scale is logarithmic

Each step on the pH scale is a factor of ten. Lemon juice at pH 2 is about ten times more acidic than a cola-like drink at pH 3 and a hundred times more acidic than tomato juice at pH 4. Pure water at 25 °C is neutral at pH 7.

16. Your stomach is strongly acidic

Gastric acid, mostly hydrochloric acid, typically has a pH between about 1.5 and 3.5. It helps unfold proteins for digestion and kills many microbes in food. A layer of mucus and bicarbonate protects the stomach wall from digesting itself.

17. Soap works because its molecules have two ends

A soap molecule has one end that is attracted to water and another that is attracted to oils. The oily ends bury themselves in grease, while the water-loving ends face outwards, so the grease is wrapped up in tiny balls called micelles and rinsed away.

18. Food browning is chemistry, not burning

The brown crust on bread, roasted coffee and seared meat comes from the Maillard reaction between sugars and amino acids at high temperatures. It creates hundreds of flavour compounds. Caramelisation, the browning of sugar on its own, is a separate reaction.

19. Catalysts speed reactions without being used up

A catalyst offers a reaction an easier path, lowering the energy needed to get started. The catalytic converter in a car uses metals such as platinum and palladium to turn toxic gases into less harmful ones. Enzymes are the catalysts of living things.

20. Salt lowers the freezing point of water

Dissolved salt gets in the way of water molecules locking into ice crystals, so salty water freezes below 0 °C. That is why roads are salted in winter and why seawater freezes at about −2 °C.

21. Water is an unusually good solvent

Water molecules are polar, with a slightly negative oxygen end and slightly positive hydrogen ends. These charged ends pull apart salts and surround many other molecules, which is why so much chemistry, including the chemistry of life, happens in water.

22. Ice floats because it is less dense than water

Most substances are denser as solids, but water expands by about 9 percent when it freezes because its molecules lock into an open hexagonal lattice. Ice therefore floats, and lakes freeze from the top down, which lets fish survive under the ice in winter.

23. Batteries are controlled chemical reactions

A battery separates a chemical reaction into two halves, so electrons can only travel from one side to the other through your device. When the reacting chemicals are used up, the battery is flat. A rechargeable battery reverses the reaction when current is pushed back through it.

24. Bananas are slightly radioactive

Bananas contain potassium, and a small fraction of all natural potassium is the radioactive isotope potassium-40. The dose from eating a banana is tiny, far below what you receive every day from natural background radiation. Your own body contains potassium-40 too.

Energy and forces

Physics underpins every other science. These facts are covered in more depth in our physics concepts article; here are the ones that matter most for the rest of this list.

25. Energy is never created or destroyed

Energy only changes form: chemical energy in food becomes movement and heat in your muscles, and sunlight becomes chemical energy in plants. This conservation law is one of the most thoroughly tested ideas in science.

26. Heat always flows from hot to cold

Left alone, a hot cup of tea cools to room temperature and never spontaneously heats up. This one-way direction is described by the second law of thermodynamics and the idea of entropy, and it is why refrigerators need power to pump heat out.

27. Light from the Sun is about eight minutes old

Light travels at 299,792,458 metres per second in a vacuum. At the Sun's average distance of about 150 million kilometres, sunlight takes roughly 8 minutes and 20 seconds to reach Earth. You always see the Sun as it was a few minutes ago.

28. Sound needs something to travel through

Sound is a vibration passed from particle to particle, so it cannot cross a vacuum. In air at 20 °C it travels about 343 metres per second, much slower than light, which is why you see lightning before you hear thunder.

29. Gravity pulls everything equally fast

Without air resistance, a hammer and a feather fall at the same rate. Apollo 15 astronaut David Scott demonstrated this on the Moon in 1971 (NASA). On Earth, air resistance slows the feather down.

30. Lightning is hotter than the surface of the Sun

A lightning bolt can heat the air around it to about 50,000 °F (roughly 28,000 °C), around five times hotter than the Sun's surface. The air expands so fast that it creates the shock wave we hear as thunder (US National Weather Service).

Life and living things

All living things share a common chemistry and a common ancestry (OpenStax Biology 2e).

31. All living things are made of cells

From bacteria to blue whales, every organism is made of one or more cells, and every cell comes from an existing cell. This cell theory, developed in the 1800s, is one of the foundations of biology. Viruses are not cells and cannot reproduce on their own, which is why biologists debate whether they are alive.

32. All known life uses the same genetic code

Bacteria, mushrooms, oak trees and humans all store genetic information in DNA and read it using nearly the same code: three-letter "words" of DNA bases that each stand for an amino acid. This shared code is strong evidence that all life descends from a common ancestor.

33. DNA is very long and very thin

The DNA in a single human cell, if stretched out, would be about 2 metres long, yet it fits in a nucleus a few thousandths of a millimetre across. It is wound around proteins called histones and folded again and again, like an extremely well-packed thread.

34. Plants build themselves mostly out of air

A tree's wood is mostly carbon, and that carbon comes from carbon dioxide taken in through its leaves, not from the soil. Using energy from sunlight, photosynthesis combines carbon dioxide and water into sugars and releases oxygen.

35. The oxygen we breathe was made by life

Earth's early atmosphere had almost no free oxygen. It accumulated after photosynthetic microbes, cyanobacteria, began releasing it, with a major rise called the Great Oxidation Event around 2.4 billion years ago. Today, ocean plankton produce a large share of the world's oxygen (NOAA).

36. Evolution by natural selection explains life's diversity

Individuals vary, some variations are inherited, and those that help an organism survive and reproduce become more common over generations. Over long periods this process, together with others such as genetic drift, produces new species. It is supported by fossils, genetics, anatomy and direct observation.

37. Evolution can be watched happening

Bacteria that survive an antibiotic pass on resistance, so resistant strains spread within years. Insects evolve resistance to pesticides, and the beaks of Galápagos finches have been measured shifting after droughts. Evolution is not only about the distant past.

38. Most species that ever lived are extinct

Estimates commonly suggest that more than 99 percent of all species that have ever existed are now extinct. Earth has seen at least five mass extinctions, including the one 66 million years ago that ended the non-bird dinosaurs.

39. Birds are living dinosaurs

Birds evolved from a group of feathered theropod dinosaurs, the same group as Velociraptor. Fossils such as Archaeopteryx and many feathered dinosaurs from China show the transition. Scientifically, a sparrow is a dinosaur.

40. Mitochondria were once free-living bacteria

The mitochondria that power your cells have their own small loop of DNA, more like a bacterium's than like the DNA in your nucleus. The evidence shows that they descend from bacteria that were engulfed by an ancestral cell more than a billion years ago, an idea called endosymbiosis.

Mushrooms look plant-like, but genetic studies show that fungi and animals share a more recent common ancestor. Like animals, fungi digest food made by other organisms, and their cell walls contain chitin, the same material as insect shells.

42. Microbes outnumber every other kind of life

A single gram of soil can contain hundreds of millions to billions of bacteria. Microbes drive the cycling of carbon and nitrogen, decompose dead matter and fix nitrogen into forms plants can use. Most species of bacteria have never been grown in a lab.

43. Some life does not depend on sunlight

Around deep-sea hydrothermal vents, microbes make food from chemicals such as hydrogen sulfide instead of sunlight, a process called chemosynthesis. Whole ecosystems of tubeworms, clams and shrimp depend on them. Their discovery in 1977 changed ideas about where life can exist.

44. Octopuses have three hearts and blue blood

Two hearts pump blood through the gills and a third pumps it around the body. Their blood uses a copper-based protein, haemocyanin, to carry oxygen, which gives it a blue colour, rather than iron-based haemoglobin.

45. Viruses vastly outnumber cells in the ocean

Seawater typically contains millions of virus particles per millilitre, most of which infect bacteria. By killing large numbers of microbes every day, marine viruses help recycle nutrients through the ocean food web.

46. Ecosystems run on energy flowing one way and matter cycling

Energy enters most ecosystems as sunlight and leaves as heat; only about a tenth is typically passed from one level of a food chain to the next. Matter, by contrast, is recycled: carbon, nitrogen and water move round and round between living things, the air, the soil and the ocean.

The human body

Your body is a collection of cooperating cells, organs and microbes (OpenStax Anatomy and Physiology 2e).

47. You are made of roughly 30 trillion human cells

A widely cited 2016 study estimated that a reference adult contains about 30 trillion human cells, around 90 percent of them blood cells, mostly red blood cells (Sender, Fuchs and Milo, 2016). It is an estimate, and it varies with body size.

48. Your bacteria roughly match your own cells in number

The same study estimated about 38 trillion bacteria in and on the body, mostly in the large intestine, giving a ratio close to 1:1, not the 10:1 often quoted. By mass, they weigh only around 0.2 kilograms.

49. Your body is mostly water

Water makes up roughly 50 to 60 percent of an adult's body mass, depending on age, sex and body composition (USGS). Fat tissue contains less water than muscle, so the share varies from person to person.

50. Humans are about 99.9 percent genetically identical

Any two people share about 99.9 percent of their DNA sequence (NHGRI). The remaining 0.1 percent still amounts to millions of differences, enough to influence traits, disease risk and ancestry.

51. Red blood cells have no nucleus

Mature human red blood cells lose their nucleus, leaving more room for haemoglobin to carry oxygen. They live for about 120 days, and your bone marrow makes millions of new ones every second to replace them.

52. Your brain uses about a fifth of your energy

The brain is about 2 percent of body mass but uses roughly 20 percent of the body's resting energy. Most of that energy goes into maintaining the electrical signals that let around 86 billion neurons communicate.

53. Nerve signals are electrical and chemical

Along a neuron, a signal travels as an electrical pulse created by charged ions moving across the cell membrane. At the gap between neurons, the synapse, the message is passed on by chemical messengers called neurotransmitters.

54. Your eyes have a blind spot

Where the optic nerve leaves the eye there are no light-sensitive cells, so each eye has a small blind spot. You do not notice it because your brain fills in the gap using information from the surrounding area and from the other eye.

55. The "tongue map" is a myth

The old diagram showing sweet at the tip and bitter at the back is wrong. Taste receptors for sweet, salty, sour, bitter and umami are spread across the whole tongue. Much of what we call flavour actually comes from smell.

56. Your immune system remembers

After an infection or a vaccine, your body keeps memory B and T cells that recognise that particular germ. If it returns, they respond faster and more strongly, often before you feel ill. This immunological memory is the principle behind vaccination (CDC).

57. Antibiotics do not work against viruses

Antibiotics target structures and processes found in bacteria, such as their cell walls. Viruses do not have these, so antibiotics do nothing for colds or flu, and taking them unnecessarily helps resistant bacteria spread (CDC).

58. Bones are living tissue

Bone is constantly broken down and rebuilt by specialised cells. This remodelling repairs tiny cracks, responds to the loads you place on it, which is why exercise strengthens bones, and lets a broken bone heal.

59. Fingerprints form before birth

The ridges on your fingertips form in the womb, roughly between the 10th and 17th weeks of development, and their exact pattern is shaped by random local conditions. That is why even identical twins have different fingerprints.

60. Goosebumps are a leftover reflex

Cold or fear makes tiny muscles pull body hairs upright. In furry mammals this traps a layer of insulating air or makes the animal look bigger. In humans, with little body hair, it only produces goosebumps.

Earth

Our planet is old, layered and constantly changing (USGS).

61. Earth is about 4.54 billion years old

Radiometric dating of the oldest meteorites and Earth minerals puts the age of Earth at about 4.54 billion years, with an uncertainty of around 1 percent (USGS). Modern humans have existed for only about 300,000 of those years.

62. If Earth's history were one day, humans arrive in the last few seconds

Squeeze 4.54 billion years into 24 hours and the first life appears in the early morning, animals with hard shells around 9 p.m., dinosaurs vanish at about 11:40 p.m., and Homo sapiens appears roughly 6 seconds before midnight.

63. The ground beneath you is moving

Earth's outer shell is broken into tectonic plates that move a few centimetres a year, about as fast as fingernails grow. Over millions of years this movement opens oceans, builds mountains and causes most earthquakes and volcanoes.

64. The continents were once joined together

Around 335 to 175 million years ago, most land formed one supercontinent, Pangaea. The matching coastlines of South America and Africa, and the same fossils and rock types on both sides of the Atlantic, are evidence of this past connection.

65. Earth's core is about as hot as the Sun's surface

Earth's inner core is a solid ball of iron and nickel at an estimated temperature of roughly 5,000 to 6,000 °C, comparable to the surface of the Sun. It stays solid despite the heat because of the enormous pressure. These figures are estimates from experiments and seismic data.

66. Earthquake waves revealed Earth's interior

Nobody has drilled deeper than about 12 kilometres. Scientists know Earth has a crust, mantle, liquid outer core and solid inner core because earthquake waves bend, slow down or stop as they pass through different layers. Inge Lehmann used this method to discover the inner core in 1936.

67. Earth's magnetic field protects the atmosphere

The churning liquid iron of the outer core generates a magnetic field that deflects most of the charged particles streaming from the Sun. Where they reach the upper atmosphere near the poles, they produce auroras. The field has reversed its direction many times in Earth's history.

68. Rocks are constantly recycled

Rocks form, break down and reform in the rock cycle. Weathering turns mountains into sediment, sediment hardens into sedimentary rock, heat and pressure change rocks into metamorphic rock, and melting creates magma that cools into igneous rock.

69. Mount Everest's summit was once under the sea

The limestone near the top of Everest contains fossils of sea creatures. It formed on an ancient seabed and was pushed upwards when the Indian plate collided with Asia, a collision that is still raising the Himalayas today.

70. Seasons are caused by Earth's tilt, not its distance from the Sun

Earth's axis is tilted by about 23.4 degrees. When your hemisphere leans towards the Sun, sunlight hits more directly and days are longer, giving summer. Earth is actually closest to the Sun in early January, during the northern winter.

Water, weather and climate

Water and air move heat around the planet and shape the weather we live with (NOAA; USGS Water Science School).

71. Oceans cover about 71 percent of Earth's surface

About 97 percent of Earth's water is salty ocean water (USGS). Most fresh water is locked in ice and groundwater; rivers and lakes hold only a tiny fraction.

72. The water you drink has been around for billions of years

Water molecules are constantly recycled through evaporation, condensation, precipitation and flow. Most of Earth's water has been here since early in the planet's history, so the water in your glass has passed through clouds, oceans, ice sheets and living things many times.

73. Air is mostly nitrogen

Dry air is about 78 percent nitrogen, 21 percent oxygen and almost 1 percent argon. Carbon dioxide makes up only about 0.04 percent, yet small changes in that amount have a large effect on climate.

74. Carbon dioxide levels are the highest in human history

Measurements at Mauna Loa, Hawaii, show atmospheric carbon dioxide rising from about 315 parts per million in 1958 to above 430 ppm at its seasonal peak in 2025 (NOAA Global Monitoring Laboratory). Ice cores show levels have not been this high for at least 800,000 years.

75. Greenhouse gases keep Earth habitable

Gases such as water vapour, carbon dioxide and methane absorb heat radiated by Earth's surface and send some of it back down. Without this natural greenhouse effect, Earth's average temperature would be well below freezing. Adding more of these gases traps more heat, which is why the planet is warming (NASA).

76. Weather and climate are different things

Weather is what the atmosphere is doing over hours and days. Climate is the average and range of weather over decades. A cold week does not disprove a warming climate, just as one hot day does not prove it.

77. Clouds are made of liquid droplets or ice, not water vapour

Water vapour is invisible. Clouds form when rising air cools and the vapour condenses onto tiny particles of dust, salt or smoke, forming droplets or ice crystals small enough to stay suspended in the air.

78. Every snowflake grows according to temperature and humidity

Snow crystals are six-sided because of the shape of the ice lattice. Whether they grow as plates, columns, needles or branching stars depends mainly on the temperature and humidity they pass through as they fall.

79. Wind is air moving from high to low pressure

The Sun heats Earth unevenly, creating differences in air pressure. Air flows from high to low pressure, and Earth's rotation deflects that flow, which is why large storms spin, anticlockwise in the Northern Hemisphere and clockwise in the Southern.

80. The ocean stores enormous amounts of heat and carbon

Water has a high heat capacity, so the oceans have absorbed more than 90 percent of the extra heat trapped by greenhouse gases in recent decades. They have also absorbed roughly a quarter to a third of human carbon dioxide emissions, which is making seawater more acidic (NOAA).

81. Ocean currents carry heat around the world

Wind-driven surface currents and a slow, deep circulation driven by differences in temperature and saltiness move heat from the tropics towards the poles. The Gulf Stream is one reason western Europe is milder than places at similar latitudes in North America.

82. Tides are caused mainly by the Moon

The Moon's gravity pulls more strongly on the side of Earth nearest to it and less on the far side, stretching the oceans into two bulges. As Earth rotates through them, most coasts get two high tides a day. The Sun's pull adds or partly cancels to make spring and neap tides (NOAA).

Space

Astronomy shows how small and how connected we are (OpenStax Astronomy 2e; NASA Science).

83. The universe is about 13.8 billion years old

Measurements of the cosmic microwave background, the afterglow of the early universe, give an age of about 13.8 billion years. The universe has been expanding since then, and galaxies are on average moving apart.

84. Looking out into space is looking back in time

Because light takes time to travel, distant objects are seen as they were when the light left them. The nearest star beyond the Sun, Proxima Centauri, is seen as it was about 4.2 years ago; distant galaxies seen by the James Webb Space Telescope are seen as they were more than 13 billion years ago.

85. The Sun is a fairly ordinary star

The Sun is a middle-aged, medium-sized star about 4.6 billion years old. It shines by fusing hydrogen into helium in its core, converting about 4 million tonnes of matter into energy every second. It holds about 99.8 percent of the mass of the solar system.

86. Stars come in colours that reveal their temperature

Red stars are the coolest and blue-white stars the hottest. The Sun, with a surface temperature of about 5,500 °C, looks white from space; it appears yellow from the ground partly because the atmosphere scatters some of its blue light.

87. Venus is hotter than Mercury

Mercury is closer to the Sun, but Venus is the hottest planet, with a surface temperature around 464 °C, hot enough to melt lead (NASA). Its thick carbon dioxide atmosphere traps heat in a runaway greenhouse effect.

88. The Moon is slowly moving away from Earth

Lasers bounced off reflectors left by Apollo astronauts show the Moon is receding by about 3.8 centimetres a year (NASA). Tidal interaction transfers energy from Earth's spin to the Moon's orbit, which also very slowly lengthens Earth's day.

89. We always see the same side of the Moon

The Moon rotates exactly once for every orbit around Earth, so one face always points towards us. This "tidal locking" is common among moons. The far side is not permanently dark; it gets just as much sunlight as the near side.

90. Moon phases are not caused by Earth's shadow

Phases happen because we see different amounts of the Moon's sunlit half as it orbits Earth. Earth's shadow only falls on the Moon during a lunar eclipse, which can happen only at full Moon.

91. Astronauts float because they are falling

The International Space Station orbits only about 400 kilometres up, where gravity is still about 90 percent as strong as on the ground. Astronauts float because they and the station are falling around Earth together, constantly missing it because of their sideways speed.

92. There are more stars than grains of sand on Earth's beaches

The Milky Way alone has an estimated 100 to 400 billion stars, and there are likely hundreds of billions of galaxies in the observable universe. Rough estimates of the total number of stars far exceed rough estimates of the number of sand grains on all of Earth's beaches. Both numbers are order-of-magnitude estimates.

93. Most stars have planets

Since the first planets around Sun-like stars were found in the 1990s, astronomers have confirmed more than 6,000 exoplanets (NASA Exoplanet Archive). Surveys suggest that most stars in our galaxy host at least one planet.

94. Most of the universe is invisible

Ordinary matter, everything made of atoms, makes up only about 5 percent of the universe's total mass and energy. About 27 percent is dark matter, detected only through its gravity, and about 68 percent is dark energy, which is accelerating the universe's expansion.

95. Black holes are real and have been imaged

Black holes form when matter is squeezed so tightly that not even light can escape its gravity. In 2019 the Event Horizon Telescope released the first image of the shadow of a black hole, at the centre of the galaxy M87, and in 2022 an image of Sagittarius A* at the centre of our own galaxy.

96. A teaspoon of neutron star would weigh billions of tonnes

When a massive star collapses, its core can become a neutron star, packing more than the Sun's mass into a ball about 20 kilometres across. Its material is so dense that a teaspoonful would weigh billions of tonnes on Earth.

How science itself works

The facts above are reliable because of the way they were found (Understanding Science, UC Museum of Paleontology).

97. Science is a process, not a list of facts

Scientists propose explanations, test them against observations and experiments, and keep those that survive repeated testing. Every fact on this list is accepted because it has been checked many times by different people, and any of them could be revised if strong new evidence appeared.

98. "Theory" means something different in science

In everyday speech a theory is a guess. In science, a theory is a well-tested explanation that ties together many facts, such as the theory of evolution or the theory of plate tectonics. Theories are among the most reliable knowledge science produces.

99. Correlation is not causation

Two things rising together, such as ice-cream sales and drownings, does not mean one causes the other; both increase in hot weather. Controlled experiments and careful statistics are needed to show cause and effect, which is why medical treatments are tested in randomised trials.

100. Measurement always has uncertainty

No measurement is perfectly exact. Good science states the uncertainty, such as Earth's age of 4.54 billion years with an uncertainty of about 1 percent. Knowing how sure we are is as important as knowing the number itself.

Where to go next

If one of these facts made you curious, the best way to understand it is to test it yourself. Our list of 101 science experiments you can do at home includes hands-on versions of many of them, such as pH indicators, density layers and DNA extraction. To see how these facts were found, read 100 scientific discoveries that changed the world, and to see them at work, try 100 applications of science in everyday life. You can browse all articles in the Science category.

References

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