This dramatic simulation shows how a new moon could be born if something large—about the size of Mars, in fact—slammed into our planet. Spoiler: it causes quite a mess.
In fact, what happens is that mantle material, from beneath our planet's crust, gets blasted into space. Over time that forms into a disc, before cooling and condensing to form a satellite. (That's actually pretty much how our moon formed, too.)
The simulation is a result of work at California Institute of Technology in Pasadena, to work out exactly which kinds of collisions ultimately result in a new moon being formed. Turns out that larger planets—greater than five times the mass of Earth—create so much vapor that shockwaves form, inhibiting the production of a moon. We were lucky to get our lovely moon, then. [New Scientist]