单项选择题

When you are near a lake or a river, you feel cool. Why The sun makes the earth hot, but it can’t make the water very hot. Although the air over the earth becomes hot, the air over the water stays cool. The hot air over the earth rises. Then the cool air over the water moves in and takes the place of the hot air. Then you feel the cool air and the wind, which makes you cool.
Of course, scientists can’t answer all of your questions. If we ask, "Why is the ocean full of salty" scientists will say that the salt comes from rocks. When a rock gets very hot or very cold, it cracks. Rain falls into the cracks. The rain then carries the salt into he earth and into the rivers. The rivers carry the salt into the ocean. But then we ask, "What happens to the salt in the ocean The ocean doesn’t get more slat every year." Scientists are not sure about the answer to this question.
We know a lot about our world. But there are still many answers that we do not have, and we are curious.
A rock cracks when ______.