What is wind? You can feel it, and you can sometimes hear it, too. You can't see the wind, but you can see all kinds of plants and flowers moving in the wind. Grass moves in the wind. So do trees. What is the wind? It is moving air. What makes the air move? Think of two places. They may be close to each other, or they may be far away. The air is cold in one place. But it is hot in the other. Warm air pushes up high into the sky. It goes higher than the cold air. The colder air then moves into the place where the warmer air used to be. The moving air is wind. Winds move clouds in the sky. A nice day can turn cloudy in a short while. Sometimes the clouds are big and dark. Big, dark clouds hold a lot of water. Soon it begins to rain. High winds can make a storm come fast. They can make a lot of noise, too. Sometimes they sound like a train going by. Very high winds have picked up big things like cars and trucks from the ground. Some have even picked up houses! But most winds are much lighter. You can have fun in a lighter wind ff you have a kite. Do you know how to fly one? Just let the wind take the kite high into the sky. According to the passage wind is ______.