There are two fish in a tank.

There are two fish in a tank. One fish turn to the other and says, "How the heck do you drive this thing!?"

This joke has multiple levels of humor, but at its core it is another example of a pun. The initial assumption of the listener is that the fish are in a fish tank, however after hearing the punchline the listeners are supposed to realize that the fish are actually in a military tank (the type of tank that you could drive).

The joke is additionally pretty absurd - two fish driving a tank is not the sort of thing that you would expect to see. Fish do not even have the necessary body parts to drive any vehicle that was created by humans. Finally, fish can't talk.

This joke is funny because fish can't talk, they can't drive tanks, and they wouldn't be in a military tank anyway.