A rule in Physics which says that no two identical particles can be in the same state (position, for instance) at the same time. This principle only applies to fermions, not to bosons. It is usually referred to as the "Pauli Exclusion Principle", after its inventor Wolfgang Pauli.
Now, here, you see, it takes all the running you can do, to keep in the same place. If you want to get somewhere else, you must run twice as fast as that.
The Red Queen in Lewis Carroll's Through the Looking Glass