阅读短文，判断正误。

Clouds are made up of millions of very small drops of water. The drops move around all the time. When they hit each other, they join up. This is how they grow larger and heavier. When they are too heavy to stay in the air, they fall down as rain.

If it is very cold, the very small drops of water change into ice. The pieces of ice freeze together. When they get too heavy, they fall down as snow.

There are many different kinds of clouds. And they are different in shape. The thunderclouds are big and dark. Some clouds are like small white balls of cotton.