Clouds are made of ice crystals that form from water vapour carried in the air. Warm air can carry a lot of water vapour, but when the air is cold the water vapour cannot remain as a gas, and turns to ice. You can see this happening when you breathe out warm air with a lot of water vapour on a winter day. Cumulus clouds form when warm moist air rises from the ground. As you go higher the air gets colder. When you come to a height where it gets cold enough that the water vapour cannot remain as a gas, it starts to form ice crystals. The flat bottom of a cumulus cloud marks the height where that happens. It marks a particular temperature level in the atmosphere. Once it passes that height, the stream of rising air might continue to rise for a while, and then it might start to break up into little eddies, or to move sideways. As it moves it carries the ice crystals with it, and the fluffy tops of the cumulus clouds show the direction the airstream has been moving, and the way the air has been welling and eddying.