Gravity is a force of attraction between any two objects that have mass and a certain distance between them. Gravity is the reason you can sit in your desk and not go floating off into the air. Gravity was discovered by Sir Isaac Newton, noted mathematician and scientist. Before this, people only knew things fell and accelerated rapidly towards the Earth. They did not know why things did this. Newton explained that objects that have mass exert a force of attraction on each other. The moon orbits the Earth because the Earth has mass and the moon has mass, and they exert a force of attraction on each other. If this were not the case, the moon would go flying off into space, and would not stay in orbit around the Earth. Objects with large mass, like the Earth, have a strong force of gravity. The other thing that affects gravity is the distance between the two objects. If you step on the bathroom scales here on Earth, the scale gives you a number which represents how much you weigh here on Earth. If we put you on the space shuttle and get you away from the Earth, if you step on the scale again, you get a much lower number, because the distance between you and the Earth has increased.