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Science

How the physical world works, from atoms and light to stars, black holes and the history of life. 62 explainers

• Why do habits form, and how do you change one?

  Repeating a behaviour in a stable context builds an automatic cue-to-action link in the brain, so the cue triggers the behaviour before you deliberate. Changing one works best by altering the cue or the environment rather than relying on willpower at the moment of temptation.

• Why do onions make you cry?

  Cutting an onion breaks cells and mixes chemicals that are normally kept apart. An enzyme called alliinase makes an unstable compound, and a second enzyme, lachrymatory-factor synthase, converts it into syn-propanethial-S-oxide, a volatile gas that irritates the eye and triggers tears.

• Why does bread rise?

  Yeast eats sugars in the dough and gives off carbon dioxide. The gas cannot escape because gluten, a stretchy protein network formed when flour meets water and is worked, traps it in thousands of tiny bubbles. In the oven those bubbles expand and then set.

• What actually happens when you cook meat?

  Heat unwinds muscle proteins so they clump and squeeze out water, turning the meat firm and opaque. Tough connective tissue slowly melts into gelatin above roughly 160F. On a dry, hot surface, sugars and amino acids react to build the brown crust and its flavor.

• Why do orchestras tune to the same note?

  Because pitch is relative, not absolute. Instruments must agree on one reference frequency or their intervals clash. The convention is A above middle C, standardised at 440 hertz, sounded by the oboe because its reedy tone cuts through and its pitch is hard to adjust.

• What do vitamins and minerals do in your body?

  Vitamins and minerals are micronutrients your body needs in tiny amounts to run its chemistry. They mostly work as helpers that let enzymes function, build tissues like bone and blood, and keep nerves, immunity, and metabolism working. Without them, specific systems fail.

• How does the sense of smell work?

  Odor molecules drift into your nose, dissolve in mucus, and bind to specialized receptor proteins on nerve cells high in the nasal cavity. Those cells fire signals to the brain's olfactory bulb, which the brain reads as a distinct smell.

• How does breathing work?

  Breathing moves air in and out of your lungs mainly by changing pressure. Your diaphragm and rib muscles enlarge the chest, dropping pressure so air flows in; when they relax, the chest shrinks and air flows out. Blood then swaps oxygen for carbon dioxide.

• How do muscles work?

  Muscles work by contracting—shortening to pull on bones. Inside each fiber, protein filaments called actin and myosin grab and slide past one another, powered by ATP and triggered by nerve signals, converting chemical energy into mechanical force and movement.

• How do we know how old the Earth is?

  Radiometric dating: certain elements decay into others at fixed, known rates, so the ratio of parent to daughter atoms in a rock acts as a clock. Meteorites that formed alongside Earth pin the planet's age at about 4.54 billion years.

• What causes the northern lights?

  The northern lights appear when charged particles from the Sun, funneled by Earth's magnetic field toward the poles, slam into oxygen and nitrogen high in the atmosphere. Those gas atoms absorb the energy and release it as glowing colored light.

• What are comets, asteroids, and meteors?

  They are three kinds of solar-system debris. Asteroids are rocky bodies orbiting the Sun; comets are icy bodies that grow glowing tails near the Sun; meteors are the streaks of light seen when small fragments burn up in Earth's atmosphere.

• What is a cell, the building block of life?

  A cell is the smallest unit of life: a membrane-bound package of chemistry that can take in energy, respond to its surroundings, and reproduce. Every living thing, from bacteria to blue whales, is made of one or more cells.

• Why is perpetual motion impossible?

  Perpetual motion machines are impossible because they violate the laws of thermodynamics. A machine cannot create energy from nothing (the first law) or run forever without loss, since friction and other real processes always dissipate usable energy as heat (the second law).

• What are gravitational waves?

  Gravitational waves are ripples in spacetime — tiny stretches and squeezes of space itself — produced when massive objects accelerate, such as two black holes spiraling together. They travel at the speed of light. Einstein predicted them in 1916; LIGO first detected them in 2015.

• What is fire?

  Fire is not a substance but a self-sustaining chemical reaction: rapid oxidation of a fuel that releases heat and light. The glowing flame you see is hot gas and soot particles emitting energy as fuel molecules break apart and recombine with oxygen.

• How does soap actually clean?

  Soap molecules have two ends — one that grips water, one that grips grease. They surround oily dirt, breaking it into tiny droplets that water can rinse away. Without soap, oil and water simply refuse to mix.

• Why does ice float?

  Ice floats because water is unusual: as it freezes, its molecules lock into an open, six-sided lattice held apart by hydrogen bonds, making solid ice roughly 9% less dense than liquid water. Less dense material floats on denser material.

• Why do we get solar and lunar eclipses?

  Eclipses happen when the Sun, Earth, and Moon line up so precisely that one casts its shadow on another. In a solar eclipse the Moon's shadow falls on Earth; in a lunar eclipse Earth's shadow falls on the Moon.

• What causes the ocean tides?

  Ocean tides are caused mainly by the Moon's gravity, with the Sun contributing about half as much. Because the Moon pulls harder on the near side of Earth than the far side, the ocean stretches into two bulges that Earth rotates through.

• How does your body clock (circadian rhythm) work?

  Your circadian rhythm is a roughly 24-hour internal cycle run by a master clock in your brain's hypothalamus, called the suprachiasmatic nucleus. It runs on genes that switch each other on and off, and daylight resets it each morning.

• How do your kidneys work?

  Your two kidneys filter your entire blood supply about 40 times a day. Roughly a million tiny units called nephrons strain out waste and excess water, reclaim what your body needs, and send the rest out as urine.

• How do your eyes see color?

  Color vision comes from three types of cone cells in your retina, each tuned to long, medium, or short wavelengths of light. Your brain compares how strongly they fire to build every hue you perceive.

• How does the heart pump blood?

  The heart is a fist-sized muscular pump with four chambers. Its right side sends oxygen-poor blood to the lungs; its left side pushes oxygen-rich blood to the body. One-way valves and the heart's own electrical pacemaker keep every beat flowing forward.

• How does digestion actually work?

  Digestion breaks food into molecules small enough for your body to absorb. Teeth and stomach churn it; enzymes and acid dismantle carbohydrates, proteins, and fats; the small intestine absorbs the nutrients into your blood, and the large intestine reclaims water before waste leaves.

• What is the periodic table really telling us?

  The periodic table arranges the elements by atomic number — the count of protons in each atom. Its columns and rows reveal that chemical properties repeat at regular intervals, because those properties are set by how an atom's electrons are arranged.

• Why is carbon the element of life?

  Carbon is the element of life because each atom forms four strong, stable bonds, letting it build the huge, varied molecules—proteins, DNA, sugars, and fats—that living chemistry needs. No other common element combines that versatility with such durability.

• What is fermentation?

  Fermentation is how cells extract energy from sugars without using oxygen. Microbes like yeast and bacteria break glucose into simpler compounds—alcohol, lactic acid, or carbon dioxide—capturing a little energy and leaving behind the byproducts we use in food and drink.

• What are acids and bases?

  Acids are substances that donate hydrogen ions (protons) in water, while bases accept them or release hydroxide ions. Acids taste sour and turn litmus red; bases feel slippery and turn litmus blue. The pH scale measures how acidic or basic a solution is.

• What is radioactivity?

  Radioactivity is the process by which unstable atomic nuclei shed excess energy by emitting particles or radiation—as alpha, beta, or gamma rays—and transform into more stable atoms, often becoming a different element entirely.

• How does magnetism work?

  Magnetism is a force created by moving electric charge. Inside atoms, electrons act like tiny spinning magnets; when many align in the same direction, their fields combine into the push or pull we can feel.

• Why is the sky blue, and why are sunsets red?

  Sunlight contains all colors; air molecules scatter short blue wavelengths far more than long red ones, so the daytime sky glows blue. At sunset, light travels through much more air, the blue scatters away, and mostly red and orange reach your eyes.

• Why do our bodies age?

  Aging is the gradual buildup of molecular and cellular damage that our repair systems can't fully fix. Over time, DNA errors, worn proteins, and worn-out cells accumulate, so tissues work less well and the risk of disease climbs.

• Why does Earth have seasons?

  Earth's axis is tilted about 23.5 degrees relative to its orbit. As the planet circles the Sun, each hemisphere leans toward the Sun for half the year and away for the other half, changing how directly sunlight strikes it.

• What is a volcano?

  A volcano is an opening in a planet's surface where molten rock, gas, and ash from deep inside escape upward. Over time, the erupted material piles up into the familiar mountains we also call volcanoes.

• What is an atom made of?

  An atom is a tiny, dense nucleus of protons and neutrons surrounded by a cloud of electrons. The protons and neutrons are themselves built from smaller particles called quarks, bound together by the strong force. Almost all of an atom is empty space.

• How do stars live and die?

  Stars are giant balls of gas that shine by fusing hydrogen into helium in their cores, balancing gravity's inward pull against outward radiation pressure. When they run out of fuel they die — small stars fade into white dwarfs, massive ones explode as supernovae.

• How did humans evolve?

  Humans evolved over roughly six to seven million years from a common ancestor shared with chimpanzees. Through natural selection acting on heritable variation, our lineage became upright, big-brained, and toolmaking, with Homo sapiens emerging in Africa about 300,000 years ago.

• What is quantum mechanics?

  Quantum mechanics is the physics of matter and energy at atomic and subatomic scales, where quantities like energy come in discrete packets and particles behave as spread-out waves of probability rather than tiny solid balls with fixed positions.

• How does electricity work?

  Electricity is the flow of electric charge, usually electrons, driven by a voltage difference. When a conductor links a high-voltage point to a lower one, charges drift through it, carrying energy that becomes light, heat, or motion.

• What is the Big Bang?

  The Big Bang is the leading model for how the universe evolved: about 13.8 billion years ago everything was hot, dense, and compact, and space itself has been expanding and cooling ever since. It describes an expansion of space, not an explosion within it.

• What is the theory of relativity?

  Relativity refers to Einstein's Special and General theories.

• What is a galaxy?

  A galaxy is a massive, gravitationally-bound system of stars, gas, dust, dark matter, and other components.

• What is a black hole?

  A black hole is a region of spacetime with gravity so intense that nothing – not even light – can escape from it.

• How does sound travel?

  Sound propagates as a mechanical pressure wave through a medium (air, water, solids).

• What is consciousness?

  Consciousness is the subjective awareness of experiences or qualia – essentially everything you experience.

• What is probability?

  Probability quantifies uncertainty by assigning numbers (0 to 1) to how likely events are to occur.

• How does a microscope work?

  A microscope magnifies tiny objects using a system of lenses (or electron beams).

• What is the scientific method?

  The scientific method is a systematic procedure for gaining knowledge about the natural world through empirical evidence.

• How do fossils form?

  Fossils form when organic remains are quickly buried and mineralized.

• How do earthquakes happen?

  Earthquakes occur when stress on geological faults is suddenly released.

• What is entropy?

  Entropy is a measure of a system's disorder or the number of microscopic configurations corresponding to its macroscopic state.

• What is light?

  Light is electromagnetic (EM) radiation.

• What is energy?

  Energy is the capacity to do work or produce heat.

• What is time?

  Time is the fourth dimension woven into the fabric of spacetime.

• What is gravity, really?

  In modern physics, gravity is not a "force" in the usual sense but the effect of mass–energy curving spacetime.

• How does the immune system work?

  The immune system is the body's defense network against pathogens.

• What is a pandemic?

  A pandemic is an epidemic of an infectious disease that has spread across a very large region (typically worldwide), affecting a substantial proportion of the population.

• How do antibiotics work?

  Antibiotics exploit fundamental differences between bacteria and human cells.

• How does evolution work?

  Biological evolution is change in the genetic composition of populations over generations.

• What is DNA and how does it encode life?

  DNA (deoxyribonucleic acid) is the hereditary molecule carrying biological instructions in all living cells.

• How does the human brain store memories?

  Memory in the brain arises from changes in neural circuitry.