That was a joke, because the one frog thought he was being challenged and had to answer a clever question. But when he thought about it, he realized there was no clever answer, and the pun was “Ribbit me this.” It’s a pun because “ribbit me this” sounds like “riddle me this.” The joke is the frog was thinking in a very serious manner and trying to figure out the answer, when really it was just a pun. Which is funny.