What is precum and when/how does it happen?

Some sperm get in: A guy's sperm are trained and ready to go at a moment's notice, while they sit waiting in the epididymis just outside each testicle. Sometimes, enough sperm get out into the "precum", but all you need is one single sperm to fertilize the egg and start a new baby. So, occasionally, probably rarely, one can get pregnant from "precum" (which is anything that comes out before the actual orgasm).