A plane takes off from an airport. A girl sits by the window and watches the houses become smaller. Soon the plane is flying above the clouds.

Why do many planes fly so high? One reason is that the air is thinner high above the ground. This means the plane can move forward with less effort and may use less fuel.

Flying high can also help a plane avoid some bad weather. But it is not always a smooth flight. There can still be strong winds and moving air.

The sky is very cold at that height. There is also too little air for people to breathe normally. Inside the plane, special systems keep the air safe and the temperature comfortable.

The girl looks at the clouds below her. They look like white mountains. A little later, the plane begins to go down toward its next airport.

The people on the plane look out of the window. The houses look very small. Up high, the air is thin. The plane has strong wings and engines to help it fly safely.