AI picks
Chosen for how satisfying the answer turns out to be
Why is 0.1 + 0.2 not exactly 0.3?
One tenth cannot be written exactly in binary, any more than one third can be written exactly in decimal — so the computer stores a very close approximation and the small errors show up when you add them.
Why does 0.999... equal exactly 1?
Because the notation does not mean a number creeping toward one — it means the value the sequence of finite pieces converges to, and that value is exactly one.
Why do 23 people give a 50% chance of a shared birthday?
Because you are not comparing 23 birthdays against 365 days — you are comparing 253 pairs of people against each other, and 253 chances at a 1-in-365 event is a coin flip.
Why can't you divide by zero?
Division asks what you must multiply by to get back where you started, and multiplying by zero destroys that information — so the question either has no answer or has every answer.
Why is the sky blue?
Air molecules scatter short-wavelength light far more strongly than long-wavelength light, so blue arrives at your eye from every direction of the sky while red passes straight through.
Why do astronauts float if gravity is still there?
They are not beyond gravity — they are falling, continuously, and so is their spacecraft, so nothing pushes back against them.
Why do we have leap years?
A year is not a whole number of days, so the calendar has to be given an extra day now and then or the seasons slowly drift away from the dates.
Why do mirrors flip left and right but not up and down?
They do not flip left and right at all — they flip front and back, and you are the one who mentally rotates yourself to face your reflection.
Why does water make a pan fire worse?
The oil is hotter than boiling water by hundreds of degrees, so the water flashes into steam instantly and blasts the burning oil upward and outward as a fireball.
Why can't a website tell you your forgotten password?
A well-built site never stored your password in the first place — only a one-way scramble of it, which can be checked against but cannot be turned back.
Why does ice float on water?
Freezing forces water molecules into an open framework with gaps in it, so the same molecules take up about a tenth more room as ice than as liquid.
Why don't antibiotics work on colds?
Antibiotics attack machinery that bacteria have and viruses do not — and a cold is a virus, which has almost no machinery of its own to attack.
Why don't birds get electrocuted on power lines?
Electricity only flows when there is a voltage difference across something, and a bird touching one wire has both feet at the same voltage, so there is nothing to push current through it.
Why are airplane windows round?
Sharp corners concentrate stress into a single point, and at cruising altitude a pressurised cabin pushes hard enough on that point to tear the fuselage open.
Why do cars have crumple zones?
Making the ends of the car fold on impact stretches the crash over a longer moment, and a longer moment means a smaller force on the people inside.
Why do websites need cookies?
The web was designed to forget you between every click, so a cookie is the note the site hands you to carry back, because it has no other way to know you are the same person.
Why does braking distance quadruple when you double your speed?
Stopping means getting rid of kinetic energy, and that energy grows with the square of speed — so twice as fast is four times as much energy to shed.
Why do computers use binary?
Because it is far easier to build a component that reliably tells apart two states than ten, and reliability is what matters when you are switching billions of times a second.
Why do carbon monoxide alarms matter when smoke alarms already exist?
Carbon monoxide has no smell, no colour and no smoke, and it binds to your blood hundreds of times more readily than oxygen — so nothing you can sense tells you it is there.
Why do websites have names instead of numbers?
Machines route by number, people remember words, and the domain name system is the enormous distributed directory that translates between the two on every click.
Why can a paracetamol overdose be deadly even when you feel fine?
The damage is done by a chemical your liver makes while breaking the drug down, and it destroys liver cells for a day or more before you feel anything at all.
Why do bridges have expansion joints?
Steel and concrete get longer when they warm up, and a bridge with nowhere to put that extra length will either buckle or tear itself apart.
Why can't anything travel faster than light?
The limit is not about light — it is the maximum speed at which any cause can reach any effect, and light simply happens to travel at it.
Why do tyres have treads?
Not for grip — a smooth tyre grips a dry road better. The grooves exist to give water somewhere to go, so the tyre can still touch the road in the rain.
Why are arches so strong?
An arch turns a downward load into a sideways squeeze, and stone is enormously strong in squeeze and almost useless in pull.
Why are rockets launched eastward?
The Earth's surface is already moving east at up to about 1,670 km/h at the equator, and launching in that direction means the rocket starts with that speed for free.
Why can't websites read files on your computer?
Browsers run every page inside a deliberately powerless sandbox, because the alternative — letting arbitrary code from strangers touch your filesystem — would make visiting a link as dangerous as running a downloaded program.
Why do skyscrapers sway in the wind?
Because building one stiff enough not to sway would be absurdly expensive and no safer — swaying is how a tall building sheds wind energy instead of resisting all of it.
Why do planes fly so high?
Thin air means less drag for the same lift, so an airliner burns far less fuel up high — and the altitude also buys smoother air, weather avoidance and time to react if something fails.
Why do bruises change colour?
A bruise is blood spilt under the skin, and the colours are the stages your body's clean-up crew takes it apart in.
Why do photos of propellers look bent?
Most digital cameras do not capture the whole image at one instant — they scan it line by line from top to bottom, so anything moving fast is recorded at a slightly different position in every row.
Why do CPUs have cache?
Main memory is so slow relative to a modern processor that without a small, very fast copy of recently used data sitting next to the core, the processor would spend most of its life waiting.
Why do we get goosebumps?
A tiny muscle at the base of every hair yanks the hair upright when you are cold or startled — useful if you are covered in fur, and left over in us because we are not.
Why is the ocean salty?
Rivers carry dissolved rock into the sea, and then only the water leaves again — evaporation takes the water and leaves the salt behind, over billions of years.
Why do SSDs wear out?
Every write forces electrons through a thin insulating layer, and each crossing damages that insulation slightly until the cell can no longer reliably hold a charge.
Why do onions make you cry?
Cutting the onion mixes two chemicals it kept apart, and the reaction makes a volatile gas that turns mildly acidic on the wet surface of your eye.
Why does Wi-Fi get slower through walls?
Walls absorb and reflect part of the radio signal, and rather than simply arriving quieter, a weakened signal makes your router switch to a slower, more repetitive way of encoding data so it can still be understood.
Why do phishing emails still work on careful people?
They do not target carelessness — they target context, arriving at a moment when the request they make is exactly what you were expecting.
Why do we get a fever when we are ill?
A fever is not the infection overheating you — it is your body deliberately raising its own thermostat, because most of your defences work better warm and most invaders work worse.
Why does bread go stale faster in the fridge?
Staling is starch slowly re-crystallising rather than bread drying out, and that crystallisation runs fastest at exactly the temperature a fridge holds.
Why do batteries degrade?
Every charge cycle drives slow side reactions that permanently consume lithium and thicken an insulating film on the electrodes, so a little capacity is lost each time and never comes back.
Why is English spelling so irregular?
The spelling was fixed by printing just as the pronunciation was in the middle of changing, and it has been carrying borrowed words from other languages ever since without adapting any of them.
Why do leaves change colour in autumn?
The yellows and oranges were there all summer, hidden under green — and the tree dismantles the green deliberately to recover the nitrogen locked inside it before dropping the leaf.
Why can't you tickle yourself?
Your brain predicts the sensations your own movements will produce and quietly turns them down, so a self-administered tickle arrives already expected and therefore not ticklish.
Why does saving early beat saving more later?
Money you invest early spends longer earning returns on its own returns, and that compounding grows with time in a way that later contributions cannot catch up with.
Why do bank transfers take days to settle?
The message that money is coming arrives in seconds, but the banks only actually exchange the money in scheduled batches, and the gap between the promise and the exchange is the delay you experience.