In a small town, a law was passed. There is only one barber in the town.
The town council has decreed that every man in town has to be clean shaven. No facial hair of any kind is allowed. The only barber in town must, under penalty of law, shave the face of all men who do not shave themselves, and ONLY those men. As a man, you have two choices:
Get shaved by barber.
If a preacher wants to shave himself, he will do so. The barber will not be allowed to shave the preacher, because hte preacher is willingly going to shave himself.
Who shaves the barber?