Suppose there is a island with just one male barber. On this island, every man keeps himself clean-shaven by doing only one of two things:
Shaving himself, or
going to the barber.
Another way to state this is:
The barber shaves all and only those men on the island who do not shave themselves.
All this seems perfectly logical, until we pose the paradoxical question:
Who shaves the barber?
Oh, Russel.