Every object has weight. But the center of its weight is not always in the middle of the object. If the top of an object is heavier than its bottom, it will fall over easily. We say that its center of gravity is high. But if the bottom is heavier, it won’t fall easily. Then we say that its center of gravity is low. Look at the first picture. This empty box must fall over, because its center of gravity (C) is not above the line of its bottom (AB). Now look at the second picture. There is some heavy iron in the bottom of this box. It will not fall, because its center of gravity is above its bottom.  小题1:If the bottom of an object is ______ its top, it will fall over easily.
 小题3:The main idea of the passage is _________.
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