Paper needs no glue to hold together. Its fibers are made mostly of cellulose and hemicellulose. In water, these form molecular links between neighboring fibers. When the water dries off, the fibers stay bonded. So most paper grades need no added glue, though strength additives can be put in. Each fiber in paper is a tiny tube. It is about 100 times as long as it is wide. Wood fibers from conifers are roughly 3 to 6 millimeters long. Wood fibers from hardwoods are about 1 millimeter long. Cotton fibers can be an inch or more. That helps explain why cotton and linen make such strong paper.

Cellulose makes up different shares of different plants. It is about 90 percent of cotton fiber, 40 to 50 percent of wood, and roughly 57 percent of dried hemp. That is why cotton is such a pure papermaking fiber. Wood has to have its lignin and other parts removed first. Modern paper machines can be about 150 meters (500 feet) long. They make a sheet 10 meters wide. They can run at more than 97 kilometers an hour (60 miles an hour). A stream of wet pulp becomes a fast-moving sheet of paper. At the far end of the mill, it is wound up into large rolls.

Cai Lun is traditionally called the inventor of paper. But paper-like material is older than he is. Archaeologists have found paper-like fragments from the 2nd century BCE in China. So his work is better described as an improvement. It made paper cheap and widespread. It was not the first invention. Papyrus and paper are different things. Papyrus is made by laying thin strips of the pith of a reed side by side. A second layer goes across them, and they are pounded together with a mallet. Paper is made from fibers that have been broken apart. They are then re-formed from a watery slurry.

The word paper comes from the Latin word papyrus. That comes from the Greek word for the papyrus plant. Even so, the two are separate inventions. Papyrus grew mainly in Egypt. It took more work and money to make than paper. It was fragile and could break apart in damp conditions. Paper could be made almost anywhere there were plant fibers and water. How many trees make a ton of paper depends on how it is pulped. One estimate uses a mix of softwoods and hardwoods about 12 meters (40 feet) tall. It is an average of 24 trees for 0.9 tonne (1 ton) of printing and writing paper by the kraft process. But only about 12 trees make a ton of mechanical pulp or newsprint, because almost all the wood becomes fiber.

Paper found everyday jobs early. The Song dynasty government of China was the first to issue paper money, in the years 960 to 1279. Earlier, in the Tang dynasty, paper was folded and sewn into square bags to keep the flavor of tea. By the 7th century, paper had reached Japan from China by way of the Korean Peninsula. Wasps and hornets were making paper long before people. Social wasps build their nests from plant fiber, mostly wood pulp. Hornets build theirs by chewing wood into a papery pulp. One history says Cai Lun's method, using rags and other plant fibers in 105 CE, was inspired by wasps and bees.

Words to know

• cellulose: plant fiber stuff
• papyrus: old reed paper
• slurry: thin wet mix
• mallet: wooden hammer