Another mathy one. Not laugh out loud hilarious but I like it:
One alien says to another, "Look, there are 11 zorgaks over there."
The other alien replies, "No, I clearly see 23 of them."
The first says, "Oh, I see. You must use base 4 on your planet. Here we use base 10."
The second then says, "I use base 10 also. What the hell is a 4?"