Tukuk-OSScience, in Plain Language
The ideas behind everything — accurate, short and free
Seventeen core ideas that school curricula usually scatter across five subjects — collected here in plain language, each in three short paragraphs. Read one a day; they stand alone. Pair them with the hands-on experiments to see the ideas move.
The seventeen ideas
Gravity — the shape of fall
Gravity is the mutual attraction of anything with mass. Earth pulls you, you pull Earth — with an equal force, but Earth's enormous mass makes its side of the effect invisible.
Newton described it as a force between masses; Einstein showed it is actually the curvature of spacetime itself — planets orbit the Sun because they follow the curved path the Sun's mass makes in space.
Weight = mass × gravitational acceleration (9.8 m/s² on Earth). That is why your mass is unchanged on the Moon, but your weight drops to one sixth there.
Energy — one currency, many wallets
Energy changes form constantly but the total never changes — the first law of thermodynamics. Food is stored chemical energy; your body spends it as motion and heat.
A moving car's kinetic energy grows with the square of speed: double the speed, quadruple the crash energy. That single square law explains most of road-safety design.
In every real conversion some energy ends up as low-grade heat nobody can reuse — entropy, the reason perpetual-motion machines are impossible.
Matter — solids, liquids, gases and beyond
What you call a state of matter is a balance between particle energy and the forces holding particles together. Heat enough and bonds lose: solid melts, liquid boils.
Temperature does not always rise while a substance changes state — boiling water stays at 100 °C while the added energy quietly breaks intermolecular bonds (latent heat).
Beyond the school four: plasma (ionised gas — the Sun, lightning, neon signs), Bose–Einstein condensates near absolute zero where atoms behave as one quantum object.
Electricity — the moving-electron economy
Electric current is a drift of electrons through a conductor — slow individually (millimetres per second), but the field that pushes them travels near light speed, so your lamp wakes instantly.
Voltage is the push, current is the flow, resistance the pinch; Ohm's law V = IR ties the three together for every circuit on Earth.
Alternating current changes direction 50–60 times a second, which lets transformers step voltage up for loss-free long-distance travel and down before it enters your home.
Magnetism — electricity's mirror twin
Magnetism is moving charge seen from another angle: currents create fields, and fields push on currents. That handshake is every motor and generator ever built.
Earth itself acts like a bar magnet, generated by churning liquid iron in its outer core. The field reverses every few hundred thousand years — the last flip was about 780,000 years ago.
That magnetic shield deflects the solar wind; without it the atmosphere would be slowly stripped away, as happened on Mars.
Light — wave, particle, messenger
Light travels at 299,792,458 m/s in vacuum — the cosmic speed limit. Colour is simply wavelength: long red waves around 700 nm, short violet near 400 nm.
Light also arrives in particles, photons — each carrying energy proportional to its frequency. Wave-particle duality is the quantum world's calling card.
Reflection (mirrors), refraction (lenses, bent straws in water), diffraction (rainbow CD grooves) and scattering (why the sky is blue) are the four everyday tricks of light.
Sound — pressure in disguise
Sound is a pressure wave riding through air at about 343 m/s. Pitch is frequency (20 Hz–20 kHz is human hearing), loudness is the wave's energy.
Sound cannot cross a vacuum — the astronauts on the Moon used radios because hammer-blow sounds had no medium to travel in the regolith.
Doppler shift: an ambulance's siren drops in pitch as it passes you because the wave crests are stretched; the same shift reveals which stars and galaxies recede from us.
Heat — the physics of "hot enough"
Heat always flows from warmer to colder; nothing spontaneously runs the other way. That one rule drives engines, refrigerators and weather.
Three transfer routes: conduction (touch — a spoon in tea), convection (moving fluid — boiling water, sea breezes), radiation (waves — Sun heat crossing empty space).
Absolute zero, −273.15 °C, is the floor: particle motion reaches its minimum and no colder state exists.
The water cycle — Earth's recycling engine
The Sun lifts roughly 500,000 km³ of water into the air each year, mostly from oceans; it condenses into clouds and falls as precipitation across land and sea.
A water molecule spends most of its life in the ocean (about 3,200 years on average), only weeks in the atmosphere — rivers empty their load in days.
The same cycle carved the Grand Canyon, filled the Great Lakes and decides your weather; every glass of water you drink has likely passed through a dinosaur.
Plate tectonics — the moving surface
Earth's crust is a cracked shell of plates floating on hot, slow-flowing mantle. They move at fingernail speed — 2–10 cm a year.
Where plates separate, new ocean floor rises (Mid-Atlantic Ridge); where they collide, mountains and trenches form (Himalaya, Peru–Chile trench); where they grind, earthquakes happen (San Andreas).
The continents were welded into Pangea about 300 million years ago and are still drifting apart; Africa splits slowly along the East African Rift.
Atoms — the mostly-empty building blocks
An atom is a nucleus of protons and neutrons surrounded by an electron cloud. If the nucleus were a marble in a stadium, the electrons would be somewhere in the stands — the rest is empty field.
The proton count (atomic number) defines the element: 6 makes carbon, 8 oxygen, 79 gold. Changing it is alchemy — and nuclear physics, which does exactly that in stars and reactors.
Chemistry is almost entirely electron behaviour: atoms bond by sharing or trading outer electrons, and every material property you touch is the result.
Chemical reactions — rearranging the furniture
A reaction breaks bonds in the starting substances and forms new ones; atoms are never created or destroyed, just reassembled — hence balancing equations.
Reactions speed up with heat, concentration, surface area and catalysts. Your body runs thousands of reactions per second at 37 °C using enzyme catalysts — chemistry labs envy that control.
Combustion (rapid oxidation), rust (slow one), neutralisation (acid + base → salt + water) and photosynthesis (energy-storing build-up) are four classics.
DNA and heredity — four-letter instructions
DNA is a twisted-ladder molecule whose letter order spells instructions for building proteins; the human genome holds about 3 billion letters per cell.
Discovered as a structure in 1953 by Watson and Crick with crucial unsung work by Rosalind Franklin; the code was fully read by the Human Genome Project in 2003.
Children get half their DNA from each parent; tiny letter typos (mutations) are the raw material of evolution — most are neutral, some harmful, occasionally one is useful.
Evolution — natural selection in action
Variation exists in every population; organisms produce more offspring than can survive; traits that help survival and reproduction get inherited more often. Heredity + selection + time = adaptation.
It is not "survival of the strongest" but of the best-fitted-to-this-environment; when the environment changes, yesterday's winners can lose — that is why extinction rates spike after climate shifts.
Observed directly: antibiotic resistance in bacteria, peppered moth colour shifts, and the finch-beak changes tracked for decades in the Galápagos.
The immune system — the body's intelligence war
First defences are general: skin, stomach acid, inflammation. Special forces are white cells that learn specific invaders, kill them and keep a wanted poster (memory cells) for next time.
Vaccines train that memory with a harmless preview — weakened, killed, or just a piece or an instruction-sheet (mRNA) of the germ — so the real attack meets a defence already prepared.
Smallpox, eradicated in 1980, is humanity's clearest proof the system works when vaccinated widely enough.
Radioactivity — unstable nuclei shed weight
Heavy atoms with too many protons or neutrons decay, firing off alpha particles (helium nuclei), beta particles (electrons/positrons) or gamma rays (high-energy light).
Half-life is fixed per isotope: carbon-14 takes 5,730 years, so once-living material's remaining fraction dates it up to ~50,000 years; uranium-lead clocks date the Earth itself to 4.54 billion years.
Background radiation is everywhere (rocks, cosmic rays, bananas' potassium-40); life evolved under it and repair systems handle ordinary doses.
Climate — the energy budget of a planet
Earth balances incoming sunlight against outgoing infrared. Greenhouse gases (water vapour, CO₂, methane) slow the escape of heat — a natural blanket without which Earth averages −18 °C instead of +15 °C.
Adding greenhouse gas thickens the blanket: since 1850s measurement began, CO₂ has climbed from 280 to over 420 ppm, the highest in at least 800,000 years (ice cores prove it).
Climate is the budget; weather is one day's spending. The trend shows up as warmer averages plus more energy in heat, drought, storms and sea-level rise (currently about 3.7 mm a year).