The questions you've had since you were five.
Why the sky is blue. Why ice floats. How a single rope can lift a piano. The physics you bump into every single day — each one answered plainly enough for a curious kid, and fully enough for a curious adult. You choose which.
Every answer on this page is written twice. Flip the switch and watch them all rewrite.
Why is the sky blue? (and sunsets red?)
Everyday physics, answered twice.
Sunlight is secretly every colour mixed together. Blue light is the bounciest, so it ricochets off the air in every direction until the whole sky glows blue.
At sunset the light skims sideways through far more air. All the bouncy blue gets scattered away before it reaches you, so only the warm reds and oranges survive the trip.
Every raindrop is a tiny round window that bends sunlight and fans it into colours. Millions of drops together paint an arc — but only when the sun is behind you.
Light travels in straight lines, so when something blocks it the light simply can't reach the floor behind — that dark patch is the shadow.
A mirror is so smooth that light bounces off at the exact same angle it arrived — like a perfect bank shot — so you see a tidy copy of whatever's in front of it.
Light slows down when it enters water and bends as it does — which is why a straw in a glass looks snapped in half.
A curved lens bends the light rays so they spread apart by the time they reach your eye, tricking it into seeing the object much bigger.
When water freezes its molecules lock into a roomy honeycomb full of gaps, so ice takes up more space than the water it came from — and lighter things float.
Heat makes air spread out and get lighter than the cooler air around it, so it floats upward — the same reason a hot-air balloon climbs.
Air is real stuff made of zillions of tiny molecules, and they're constantly bumping into everything. That steady drumming of hits is air pressure.
Gears let you choose your trade: an easy gear turns the wheel a little per pedal but feels light; a hard gear turns it a lot but takes more push.
Push down far from the pivot and you can lift something heavy close to it — a lever trades a long, easy push for a short, strong lift.
Loop a rope through wheels and the weight gets shared across several strands, so each pull feels lighter — though you reel in more rope.
A ramp lets you raise something by pushing it up a gentle slope instead of lifting it straight up — easier, but you travel farther.
Surfaces aren't truly smooth — up close they're rough and grippy, so they drag against each other and slow things down. That grip is friction.
Momentum is how hard something is to stop. A heavy or fast-moving thing has loads of it — which is why a rolling truck is scarier than a rolling ball.
Things are lazy: a resting object wants to stay put and a moving one wants to keep going, until something pushes it. That stubbornness is inertia.
Throw a ball and two things happen at once: it coasts forward steadily while gravity pulls it down. Together they curve its path into a graceful arc.
A swing takes about the same time for every swing, big or small. Make the string longer and it swings more slowly.
Sound is air wiggling. Something vibrates, squishes the air next to it, and that squeeze travels to your ear as a wave you hear.
Twenty down. Next on the bench: optics you can bend with your own lens, a pendulum that actually swings, and a wave you can pluck. The shelf keeps growing.