Fire a pulse of about a hundred million billion photons at the Apollo mirrors and, on a good night, one of them comes back.

The beam is a couple of kilometres wide by the time it gets there and about twenty kilometres wide by the time it returns, so almost all of it misses. Catching single photons and timing them to a tenth of a nanosecond is how the Earth-Moon distance is known to a few millimetres — the most precise measurement anyone makes of anything that far away.