Things can live on Earth because it has the right mixture of heat, air and water. Over half of the Earth is convered in water. All living things need water to stay alive. The Earth's crust is made up of pieces. The place where the pieces slide past each other is called a fault. Pieces of crust can move smoothly along a fault, but sometimes they get jammed. Jammed pieces can suddenly start to slip past one another. The land above trembles and cracks. This is called an earthquake. Big earthquakes can destroy buildings and tear up roads. As the crust moves, it slowly pushes some of the land up into tall, rocky mountains.