Scientists collect evidence, including objects they have found and things they have observed, as part of their research. Then, based on this evidence, they come up with a theory that explains how something occurs—or has occurred—in the natural world.
Scientists use theories to explain everything from how the universe began to why objects fall to the ground. In 1915, German scientist Alfred Wegener published a book to present a theory he called "continental drift." Wegener believed that the large pieces of land we call continents move over long periods of time. Even though Wegener found strong evidence to support his theory, not everyone accepted it from the start.