51. scientists believe that the first americans came from northeast asia. these people were probably hunters from what is now northern china, japan, or siberia. many thousands of years ago , they crossed over from asia to what is now Alaska. From there, they spread throughout north and south america. the first evidence for this theory was found in chili in 1936, when scientists found the teeth of some very early indians. these teeth proved to be very similar to the teeth of people in
52. we all know that monkeys are smart animals, but sometimes their intelligence is surprising and entertaining. a psychologist once wanted to see just how smart a monkey was. he hung a banana high up in a monkey's cage and placed several large boxes and a stick nearby. He wanted to see if the monkey could use the boxes and the stick to get the banana. the monkey looked at the banana, the boxes, and the stick. then it took the psychologist's hand and led him to where the banana was hanging. it jumped up onto his
shoulders and
53. unlike plastic, which is artificial, rubber is a natural product. it is made from latex, a white liquid found in certain plants, especially the para rubber tree. this tree originally came from the amazon forests in brazil. the indians of that area used the latex from the rubber tree to make status, cups, and shoes. when latex was discovered by europeans, they soon found many uses for it. for example, an englishman named Chares mackintosh invented a way to make waterproof rainwear with latex. his method is no longer used, but
even today,many people in England
54. after simple animal skins, wool is probably the oldest material used for making clothing. we do not know exactly when people started to make woolen clothing, but it was probably quite early in human history. the wool was made from the hair of whatever kind of animal people had available. most of the time these were sheep, but in some desert areas people made cloth from camel hair. in other areas, they used goat hair, and in the mountains of south america, they used the hair from llamas. all these kinds of wool have one thing in common: they protect a person's body from outside changes in temperature.
woolen clothing keeps
55. farmers in most of the industrialized countries grow cash crops today. this means that they usually grow and sell large amounts of only a few crops, such as soy, wheat, or corn.
they use the money to buy what they need for their families and farms. in the past, farming was quite different. most farmers used to grow many different kinds of crops and
also raise cows,pigs, chickens, and other animals. they sometimes sold extra farm products or animals, but