Diamonds

　When most people think of diamonds, they think of beautiful jewelry. But the beauty of diamonds is only one reason they are so ( 36 ).
　Another reason is that diamonds are the hardest natural objects on earth. Every year, 10 tons of diamonds are dug out of the ground. But only about 20% of those diamonds are large enough and good enough to be used for jewelry. The other 80% are called "industrial diamonds" and are used to create thousands of products, from eyeglasses to computer chips. Because industrial diamonds are so hard, they are used in many kinds of tools. ( 37 ), dentist's drills and machines for cutting metal and stone use industrial diamonds. Without industrial diamonds, many kinds of factories could not ( 38 ).
　Scientists have found another reason why people should be interested in diamonds --- they can ( 39 ) us many things about the history of the earth. Most diamonds were formed billions of years ago deep inside the earth. About 100 miles underground, there is a part of the earth called the mantle. Inside the mantle, very high temperatures and pressure turned a material called carbon into diamonds. During this process,other chemicals from the mantle often got caught inside the diamonds. Later, huge volcanoes, acting like elevators, brought the diamonds up from the mantle. So now scientists ( 40 ) diamonds can learn a great deal about what the mantle was like billions years ago.

No.1　　　Signs from Costa Rica's Mysterious Past

　What do you know about the country called Cota Rica? The early history of Costa Rica is less well understood than that of any other country in Central America. The ancient people who lived there left few signs of their culture. Scientists have found streets and buildings made of stone, as well as peices of pots and some artwork made of gold. From these objects, scientists try to guess how the people lived. But since the people had no written language, it is very difficult to create a clear picture of what their culture was like.
　Spain's contact with Costa Rica began with Christopher Columbus's arrival in 1502. The Spanish brought their own culture to the region. They also brought with them diseases that killed many of the native people. As a result, the knowledge of these people's cultures has been lost.
　Among the many mysteries of Costa Rica's ancient past, perhaps the most amazing are the Diquis Spheres. These are balls made of stone which were first discovered in an area called Diquis in southern Costa Rica. Thousands of these balls have been found along rivers and in ancient cemeteries throughout Diquis. Recently , more spheres were found on Cano Island, an island about 20 kilometers off Costa Rica's Paciffic coast.
　The spheres come in many different sizes. Some are as small as an orange, while others are about 2 meters tall and weigh 16 tons. But no matter what their size, all the spheres are almost perfectly round. Scientists don't know who made the spheres or what meaning they had for their creators. Researchers can't explain how they made the spheres so beautifully round.

No.2　　　Diet and the Aging Process

　Scientists are continually looking for ways to help us live longer. Some say that the length of our life is decided mainly by our genes. But other scientists are studying diet. In the past few years, studies have shown that giving mice less food improves their health. When scientists gave mice half as much food as they normally eat, the mice lived as much as 50% longer. In other studies, rats that ate less food had more energy and were more active.
　The National Institute on Aging, in the United States, is now carrying out a study on monkeys, since they are similar to human beings in many ways. The animals whose diet was cut by 30% are having fewer health problems than those eating a normal diet. The monkeys in the study are now in their mid-20s. But monkeys can live to be 35 or 40 years old in the wild. So scientists will have to wait 15 years to find out whether the monkeys on the restricted diet live longer than other monkeys.
　It is not yet clear whether eating less would have a similar effect on humans. But some people are already trying to live on a restricted diet. Many of them find it very difficult because they must be careful to include enough vitamins to stay healthy. But they also have troule with restricted diets because they enjoy eating. Most scientists say it is not worth giving up your favorite foods yet. They say people should wait until there is more evidence about the effects of restricted diets on their health.