Why shouldn't you tell a joke to an egg?

This joke plays on the double meaning of the phrase “crack up.” In one sense, “to crack up” means to burst into laughter, a common reaction to hearing a funny joke. In the context of an egg, however, “to crack” literally means to break or split, as eggshells are fragile and easily broken. The humor in the joke comes from this wordplay, where telling a joke to an egg would not only make it “laugh” (figuratively speaking) but also cause it to physically crack, as if the egg were responding to the joke as a person might, but in a manner consistent with its nature as an egg. The joke is a simple, whimsical play on words.