Everyday science
The physics and biology of an ordinary afternoon
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 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 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 time seem to speed up as you get older?
Because you judge elapsed time largely by how much you remember of it, and a familiar year lays down far fewer distinct memories than a novel one.
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 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 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 does hot water sometimes freeze faster than cold water?
Sometimes it does and sometimes it does not — and after sixty years of attempts, nobody has produced an experiment that reliably shows it or a mechanism everyone accepts.
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 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 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 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 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 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 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 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 elevators have mirrors?
Mirrors were added because complaints about slow elevators dropped sharply when people had something to look at, which was far cheaper than making the elevators faster.