The wheel arrived surprisingly late
People were weaving cloth, farming, making pottery and sailing BOATS for thousands of years before anyone built a wheeled vehicle. The wheel appears only around 3500 BC, which seems odd for something so obvious.
The difficulty was never the round part. It was the axle. A wheel needs a hole through the middle that is perfectly round, a shaft that fits closely enough not to wobble but loosely enough to spin, and both must be strong enough to carry a load and smooth enough not to grind themselves away. Without metal tools, shaping that from wood is genuinely hard — and until you have it, a wheel is just a heavy disc.
Wheels need somewhere to roll
A wheel also only earns its keep in the right conditions. Civilisations in the Americas made wheeled toys, so the idea was plainly understood, yet nothing full-sized ran on wheels. They had no horses or oxen to pull a WAGON — those arrived with Europeans — and much of the land is mountain, forest or jungle where a track for a cart would have to be cut and maintained for hundreds of miles.
So people carried loads on their backs and on llamas, along superb footpaths. The lesson runs through this whole category: a vehicle is a bargain with its surroundings. A TRAM needs rails, a FERRY needs deep water, a PLANE needs a runway, and any of them is useless without the network built around it.
Each one pushes against something
Underneath the variety, every vehicle in this grid moves by shoving something backwards. A CAR, a TRACTOR and a BICYCLE push against the ground through friction between tyre and road. A BOAT or a SUBMARINE pushes water back with a propeller. A PLANE and a HELICOPTER throw air downwards or backwards. Nothing else is available, so those are the three families.
Except one. A ROCKET has nothing to push against at all, so it brings its own: it hurls its burnt fuel out of the back at enormous speed and moves the other way. That is why a rocket works in space, where a propeller or a wing would be entirely useless.
How fast, how quickly
For most of history the fastest anyone travelled was a galloping horse. Then, in a single century, that record fell over and over: early steam TRAINS beat the horse in the 1830s, the first powered PLANE flew in 1903 at about the speed of a modern bicycle sprint, and by 1969 astronauts were returning from the Moon at around 39,000 kilometres an hour.
The sub-topics below take one machine each — cars, trucks, trains, planes, boats and a dozen more — and the newest chapter runs through most of them. Electric motors are replacing engines on the BUS, the VAN, the CAR and the SCOOTER, which is less a new vehicle than a new way of feeding the old ones.
One thing has not changed in five thousand years. Every vehicle here exists because moving something — a person, a crop, a parcel, a patient — justifies the effort of building a machine to do it, and almost every design in this category is a different answer to the question of what is being moved, and where.