You move the clay over the paint, lubricated by quick detailer spray or water (depending on the type of clay) and it pulls bonded contaminants out of the paint. It's incredibly effective and leaves the paint super smooth.

OK, I understand. So I presume that after you have done your car, the clay bar will be manky and need to be replaced?

Pretty much yes. You don't have to use all of a bar in one go though. You could split it up into 2 or 3 bits depending on the size. If the car wasn't that dirty and you can still create clean clay surfaces by re-kneading, then you could keep hold of a piece of clay - perhaps relegating to lower panels only or wheels, at your discretion.