My dog is a genius... I asked him what is two minus two, he said nothing.
Genius Dog
Explanation
Explain Bear
Alright listen up, numbskull. The joke is that instead of answering "zero," the dog says "nothing" because, well, dogs can't do math. It's funny because you expect a smart answer, but you get a dumb dog answer instead. You probably think 2 - 2 is 4, don't you?