Gravity is one of the four fundamental forces of nature, without which the universe would be unrecognizable. Gravity is the weakest of these four forces, but it is important to life on Earth and the structure of the universe. Everything that has matter generates gravity, from a grain of sand to the largest objects in the universe. That gravity pulls things together.

When it comes to gravity, the larger an object is, the stronger its force. A person creates gravity but not enough to pull objects toward him or cause things to go into orbit around him. On the other hand, a planet has enough gravity to pull objects into orbit around it. A star makes enough gravity that it can pull whole solar systems into its orbit, like ours. Our sun's gravity is so strong that it keeps an object -- Pluto -- that is roughly 3.7 billion miles away in orbit.

The Moon is in orbit around Earth. That means that it circles Earth without crashing into it or floating away. The reason the Moon can do this is the gravitational pull of our planet. No one knows for sure whether the Moon is a piece of Earth that came free before the planet cooled, a passing chunk that was caught by gravity or a conglomeration of space debris that Earth sucked in and made into a ball -- but we do know that gravity keeps it where it is.

Because it is made of matter, the Moon also has a gravitational pull, but it is not strong enough to move Earth. However, it is strong enough to move the oceans. Whenever the water on the beach recedes and comes back with the ebb and flow of the tide, the ocean is reacting to the pull of the Moon's gravity. The sun causes some tides as well.

Isaac Newton is the astronomer, mathematician and physicist who discovered the force of gravity and formulated the Universal Law of Gravitation. There is a popular story that he made the discovery when an apple fell out of a tree and hit him on the head. This story is most likely apocryphal although anything that falls on Earth -- including apples -- is subject to the planet's gravitational pull.

Without gravity, all people and other objects would go floating off into space. Gravity also keeps Earth close enough and far enough from the sun that we are not freezing or burning. Therefore, life would not have begun on Earth were it not for the force of gravity.