In the 1700s, Isaac Newton determined that the force of gravity between two objects depends on the mass of the objects and distance between them. He reasoned that things fall toward Earth because it has so much mass. Then in the 1900s, Albert Einstein worked with new discoveries, such as the fact that light always travels the same speed, and he determined a new explanation of how and why gravity works. Mass and distance still matter in Einstein’s explanation, but speed also matters. Newton’s explanation still works for objects close to Earth, but Einstein’s explanation works for all of the objects in the universe.