Hidden engineering
The deliberate decisions inside things you use without thinking
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 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 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 do keyboards use QWERTY?
It was designed around the mechanical constraints of a machine nobody uses any more, and it survived because everyone had already learned it.
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 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 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 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 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 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 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 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 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 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 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 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 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 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 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.