When the weather is hot, you go to a lake or an ocean. When you are near a lake or an ocean，you feel cool. Why？The sun makes the earth hot but it cannot 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 goes up. Then the cool air over the water moves in and takes the place of the hot air. When it moves in，you will feel the wind. And the wind makes you cool. 

Of course，scientists cannot answer all of our questions. If we ask "Why is the ocean full of salt？"，scientists will say that the salt comes from rocks. When a rock gets very hot or very cold，it cracks(断裂). Rain falls into the cracks and then carries the salt into the ocean. But then we ask，"What happens to the salt in the ocean？"The ocean does not get saltier 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 which we don't have and are curious(好奇的) about.