Scientists used to believe that the Earth and Moon were created after a planet they called Theia slammed into Earth. The collision caused the smaller Theia to break up, sending fragments into space. One of these fragments became the Moon.
Scientists disproved this theory after tests on Moon rocks revealed that the Earth and Moon were made from the same material. These days, scientists believe that Theia slammed into an older Earth 100 million years ago. Both planets mixed together to create Earth. A fragment of Earth later broke off to form the Moon.