In following the motion of a collection of identical classical particles – say billiard balls – it is always possible for us to keep track of individual particles. For example, to find out where a given particle, “number 4,” is at any later time we merely have to paint a “4” on that particle (carefully enough so that the paint won’t affect its dynamics) and then notice at the later time which particle is labeled “4.” It is not, however, possible to do this when studying a system of identical quantum mechanical particles, such as electrons. The problem is that quantum mechanical particles are too small for one to attach physical labels; they don’t have enough degrees of freedom to enable one to mark each particle differently.