But how did a rock from Earth end up on the Moon? Well, boulders from the latter often get fired towards our planet, and the reverse is also likely. As William Bottke from Colorado’s Southwest Research Institute put it to New Scientist, “Earth’s been hit by a lot of very big things. And it’s conceivable that some of those impacts have ejected material far enough away that it’s been able to escape the clutches of Earth and make it to the Moon.”