In a certain small town, there is a barber named Bill. Since Bill is the only barber in the town, he decides that he will shave all the town's residents who do not shave themselves, but (obviously) not the ones who do.
If Bill follows this rule, will he shave himself or not?
If you want to really get down to it, then:
You need to define what "shave themselves" means.
If it means "have ever shaved themselves" then he will shave himself once, then never again.
If it means "shave themselves once every x days" then he will shave himself after enough time has passed so that he is outside the definition of "shave themselves"
Bill should probably just go the next town and get a good shave and maybe a haircut, too.