The riddle
Why Don't Eggs Tell Jokes?
Cracking up two ways
An egg cracks, and a good joke can crack a listener up. Wiktionary lists both under crack up: to laugh heartily, to make someone laugh, and to break apart. The riddle asks why eggs keep quiet, and the answer is that telling jokes would make the whole carton laugh until each shell gave way.
What a shell is made of
Wikipedia says a chicken eggshell is about 95 to 97 percent calcium carbonate crystals, held together by a protein matrix. Narrow pores run through it to let gas pass, and the outermost layer is called the cuticle. A shell like that is thin enough to break on the edge of a bowl, which is why eggs make a good joke about fragile things. The riddle does not say eggs lack a sense of humor. It says they have too much of one.
An older joke about cracking
Crack also names a sharply humorous comment, a wisecrack, and people talk about cracking jokes. The 1890 collection Riddles and Their Answers, compiled by Charles B. Eaton, asks, "Why are jokes like nuts?" and answers, "Because the drier they are, the better they crack." The same verb connects jokes and food in both riddles.
Trying it with a carton
A counting riddle with eggs in it, I have 6 eggs, I broke 2, cooked 2 and ate 2, makes a good partner. Tell the joke with a straight face, then ask the group which other kitchen item would be a poor choice to tell jokes, and why.