题目内容：
根据下面资料，回答题 Our Muscles
①When we think of muscles, we might often picture a bodybuilder with big armand legmuscles. But our muscles do not have to look like that to work well.
②As a matter of fact, every time we write our name, we use the 20 different muscles in ourhand. Every time our heart beats, we use our cardiac muscle (心肌). When we eat our food, weuse our tongue muscles. We even use muscles when we open and close our eyes. In fact, every timewe move, we use some of the muscles in our body.
③A muscle is made of tiny fibers. Fiber is a type of tissue that feels a little like a rubber band.Thousands of these fibers are packed together to make a single muscle. We all have the samenumber of muscles--about 700 or so. Men and women with big muscles simply have thicker bundlesof fibers.
④There are three different types of muscles in our body. Smooth muscles are the ones that wecannot control. They work quietly to keep our body running. Also, they are the ones that help us toabsorb our food. The cardiac muscle makes the heart pump blood in and out. Skeletal muscles arethe ones that we can control. These are the muscles that we use to raise our hands, or ride ourbicycles.
⑤Even our face often uses muscles. But if we want to save our energy, try smiling instead offrowning. Remember that it takes 17 muscles in our face to smile, but 43 muscles to frown!
Muscles don' t have to be very big __________.
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