Every so often Earth's gravity grabs a passing rock a metre or two across and it goes round a few times before escaping. Two have been caught in the act.

2006 RH120 stayed for about a year; 2020 CD3 for roughly three. Models say there should be one metre-sized minimoon in orbit at any moment, but they are dark and tiny and almost impossible to find. They are not moons in the sense that the Moon is a moon, and calling them that has annoyed a number of astronomers.