The most popular theory about the origin of the Moon is that it was formed from debris breaking off the Earth in a planetary collision 4.5 billion years ago. If this were true, the Moon would be made of the same material as is found on Earth. Examination of Moon rocks shows this to be the case, although there is little iron in Moon rocks. However, this can be explained because, according to the theory, the material that formed the Moon would have come from the Earth's crust, not its iron-rich core. Thus we should accept the theory as true.