Q: What do you call an elf wearing ear muffs?

Q: What do you call an elf wearing ear muffs?
A: Anything you want. He can’t hear you!

This joke is funny because it breaks the conventions of similar jokes. It also uses something approaching antihumor to do so.

In order to fully appreciate this joke you have to have heard a joke along the lines of "what do you call a legless cow? Ground beef" before. If you hear this joke, and expect the answer to be something like that, then the answer here is entirely unexpected.

The answer is also not particularly humorous. The funniness of the joke is that the listener was thinking along other lines before the delivery of the punch line.