A new car model takes about three years or more from the first sketch until it is on the factory line. Designers go from sketches to clay models to hand-built prototypes before the factory builds a new car. They make big clay models and cut and shape the clay by hand.

Cars are built in very big factories, where people and machines make many cars every day. Cars move along a line from station to station while workers and machines add parts to them. In car factories the word marriage means the moment the car body is joined with its engine, axles and other mechanical parts.

In 1913 Henry Ford's workers built cars on a line that moved. Ford's moving assembly line first ran at his Highland Park plant in Michigan in October 1913. A rope and winch pulled the chassis of each Model T along a short track. Ford was not the first carmaker to use an assembly line. What was new in 1913 was a line that moved the car to the workers.

Ford first tried the line on a small part. A magneto took 20 minutes to build, and after the line it took five. The Model T line kept getting faster. At first it cut the build time from 12.5 hours to under 6, and within a year to 93 minutes.

A car's paint is put on in layers, not all at once. Primer makes the surface smooth and even. A base coat gives the car its color. A clear coat on top protects the color from sunlight and from air that can dull it.

Body parts are made from big rolls of steel or aluminum, which are cut into flat pieces before they are pressed into shape. Big presses squeeze the flat metal into the shapes of car parts such as doors and side panels. A press line can run up to 18 strokes a minute, and one press shop can make more than 10,000 car parts in a day. Tesla uses giant machines to cast big parts of a car body from molten aluminum in one piece.

Toyota's way of building cars rests on two ideas: just-in-time and jidoka. Toyota's just-in-time idea comes from a comment by Kiichiro Toyoda, and Taiichi Ohno later developed it into a system that controls overproduction. Sakichi Toyoda built a loom that stopped by itself when a thread broke. That idea became jidoka.

A robot called Unimate was put to work in a General Motors factory in 1961. The first Unimate robot did a job that was dirty and dangerous: it unloaded hot finished castings from a die-casting press. That was a hot job that people did not want.

A car factory cannot work without computer chips. In 2021 a shortage of chips made carmakers stop work and cancel shifts.

Over 12 million cars are recycled in the United States in a year. The steel in old cars is melted and made into new steel, which can become cans, appliances, buildings or new cars. Cars are the most recycled thing people buy, because nearly all the cars that leave the road are recycled.

Words to know

• marriage: joining step
• castings: poured metal
• overproduction: making too much
• prototypes: test models