So for some reason I decided that it was time Jackson knew a joke.
Me: Okay, when I say "knock knock" you say "who's there?"
Jackson: Who's there?
Me: Good. Knock knock.
Me: Now you say "who's there?"
Jackson: But I already did!
Me: Well, just say it again. Knock knock.
Jackson (god my mother is stupid): Who's there?
Jackson: Okay, come in.
Me: No, you say, "Lettuce who?"
Jackson (wtf?): Lettuce who?
Me: Lettuce in! Get it? It's like let us in. You see? It's a play on words.
A few days later I remembered another one that my best friend in high school used to tell me when we were high.
Knock knock. Who's there? Dwayne. Dwayne who? Dwayne the bathtub I'm dwowning.
Those are the only two knock-knock jokes I can think of.