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One way that scientists learn about man is by studying animals．In this lab, the scientists are studying the relationship between diet and health．
They are studying the relationship between the amount of food the mice eat and their health．The mice are in three groups．All three groups are receiving the same healthy food．The first group is eating one cup of food each day, the second group is eating two cups, and the third group is eating three cups．
After three years, the healthiest group is the one that is only eating one cup of food each day．The mice in this group are a little thinner。But they are more active．Most of the day, they are running, playing with one another．Also, they are living longer．Mice usually live for two years．Most of the mice in this group are still alive after three years．
The second group of mice is healthy, too．They are active, but not as active as the thinner mice．They are only living about two years．
The last group of mice is receiving more food than the other two groups．Most of the day, these mice are eating or sleeping．They are not very active．These mice are living longer than the scientists thought－about a year and a half．But they are not healthy．They are sick more often than the other two groups．
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