Gravity is a force that pulls things toward each other. It can only pull, never push. Earth's gravity keeps you on the ground and makes things fall. It is why things fall down when you drop them or throw them. Anything that has mass also has gravity. Things with more mass have more gravity. Earth's gravity pulls on the mass in your body, and that gives you weight. Gravity gets weaker with distance. So the closer two things are, the stronger their pull.

Gravity does exist in space. It keeps spacecraft in orbit around Earth. It keeps Earth in orbit around the Sun. A thing in motion stays in motion unless something pushes or pulls on it. A thing in orbit is always falling toward the planet. But it moves sideways fast enough that it never hits it. Astronauts on the space station float because they and the station are all falling together around Earth.

A falling apple is said to have inspired Isaac Newton to connect it with the motion of the Moon. Gravity keeps the Moon moving around Earth. In 1687 Newton explained that ocean tides come from the gravitational pull of the Sun and Moon on the oceans. The Moon's pull is the main cause of tides.

Albert Einstein described gravity as a curve in space that wraps around objects such as stars and planets. Gravity also pulls on light, so a massive object can bend light. A gravitational wave is an invisible ripple in space. Scientists first directly detected gravitational waves in 2015. They used LIGO.

Words to know

• mass: amount of stuff
• orbit: a path around
• tides: sea rising, falling
• LIGO: wave detector