Mountains are high places on Earth. Earth has a crust. Earth's crust moves very, very slowly. It can move up and make mountains.
Mountains are very high. Mount Everest is about 8,850 meters high. Mountains are very old, too. Some mountains are millions of years old.
Volcanoes are mountains. A volcano is a hole in Earth's crust. Under Earth's crust, there's hot rock. The hot rock comes out of the volcano. Volcanoes can be dangerous.