Some of my classes are mildly chatty. And because of that, I use the same theme joke all the time ... by just substituting in the word, "shhhh", into a sentence:
"You know what my favorite word is?" .... "shhhhhh"
"Today is brought to you by the letter, shhhhhhhh"
... when the noise level is rising.
Well. Today the noise level was rising, and I'm about to make my shhhh joke, and spontaneously I latch onto the sentence a kid just said about an odor in class. So (obviously, without thinking) I blurt, "it smells like shhhhhhhh in here."