More and more scientific experiments prove that physical exercise can reduce the dangers of some illnesses in middle-aged persons．Exercise strengthens the heart muscle，reduces blood pressure and help to prevent muscles from changing into fat．Physical exercise is just as important for children． Exercise and food affect growing speed in young lab animals．Baby mice start running as soon as they are big enough to use an exercise wheel in their cage．If they get extra food and run a lot，they will grow as much as 1.5 times bigger than normal． The same differences in growing speed might be found between active and inactive children．Physical exercise helps active children grow faster than inactive children．One experiment shows that the brains of the mice that had enough exercise weighed about 3％more than those of the mice that did not exercise The mice that exercised are much quicker to learn doing new exercise than the mice that did not exercise． The results of the experiments suppose the theory that exercise can help babies learn to talk and walk sooner than expected． The good effects of physical exercise are not limited to children and middle-aged people．Exercise continues to be an important part of our lives after we grow old．For example，people over 50 years old begin to lose calcium(钙) from their bones，which get weaker and can break easily．Physical exercise，however，helps to strengthen the bones and to prevent them from losing calcium．Of course，old people can take medicines to prevent themselves from suffering from losing calcium．But the medicines they take increase the chance of developing some kind of cancer(癌)．So physical exercise is a much safer treatment(治疗)． 小题1:From the passage we know that .
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