Second only to oxygen, silicon is the most abundant element in Earth's crust. It is found in rocks, sand, clays and soils, combined with either oxygen as silicon dioxide, or with oxygen and other elements as silicates.
Gravity is a force that attracts objects. It is also the force that makes objects fall down when you drop them. Because of gravity things on earth have a certain weight. Gravity on Earth pulls objects to the centre of the planet. Every time you throw a ball up into the air it is pulled back to Earth. This occurs in nature all the time.
Everything on Earth â including us â is held down by the force of gravity. Without gravity, we would all float off into space. It is said that Isaac Newton discovered gravity when an apple fell on his head. In order to reach space, we use a rocket-powered launcher to overcome the pull of Earthâs gravity. Other ideas have been put forward from time to time.