Coffee is one of the most popular ( 流行的 ) drinks throughout the world today. In fact, according to some estimates, over 30% of all adults in the world drink coffee at least once a day on the average. Coffee contains a kind of drug called caffeine ( 咖啡因 ). Caffeine is a chemical that stimulates ( 刺激 ) the nerves of the body. Drinking coffee tends to make people a little bit more awake — at least for a short time―because of this stimulating effect on the nervous system ( 系统 ). A cup of coffee has, on the average, about 3% caffeine in it. One story of the discovery of the coffee plant relates to this effect of caffeine. According to the story, coffee was discovered in East Africa. The story says that coffee was first found by a goat farmer named Kaldi. This was about the year 850. Kaldi was leading his animals through the mountains and the goats were stopping repeatedly to eat the plants near the path. Suddenly, some of the goats started jumping up and down in a very strange way. Kaldi figured out that the goats were acting this way because of the plants they were eating. Kaldi himself tried eating some of the green beans ( 豆荚 ) that the goats had been eating. He, too, felt the stimulating effect of the beans. Kaldi wanted to prove what had happened, so he picked some of the beans and took them back to his home village, where he told his story. The green bean got the name "Kaffa" and later "coffee" because the beans were discovered in a place called Kaffa in Africa. Then for years, people used to eat a few of the green Kaffa beans when they were in the mountains and needed extra energy to do their work. It was later found that the coffee beans could be picked and then dried until they turned brown, and then they could be stored. If the beans were dried and stored, they could be used at any time.