Why is that funny?

Our kids have been banned from knock knock jokes at the table. Most of you with kids understand why Heather and I did that. It's not that we are mean, or that we don't want to have fun together, we just want to talk and laugh about things that are actually funny. For those of you who don't have kids, or that have kids sophisticated enough to only tell jokes that make sense, the following is an example of the kinds of knock knocks that we experienced daily before instituting our rule.

Child: "Knock, knock"
Me: "Who's there"
Child: "Fork"
Me: "Fork who?"
Child: "This fork that I'm eating with!"(riotous laughter)

As all parents know, one bad knock knock joke begets another. So once it gets started it is hard to stop. Overall it has been a good decision, and we have much more "stimulating" conversations without the knock knocks. They do try other jokes, but mostly we just have fun at the table without joking. However, today Daylin tried out a new one on me that she had heard.

Daylin: "Daddy, do you know why 10 is afraid of 7?"
Me: "Um, because 7 ate 9?"
Daylin: "Yep!"
Bethany: "Why is that funny?"
Me: "Because 7, ate, 9"
Daylin: (the lights go on) "OH!"
Bethany: (no lights) "Huh?"
Me: "The number 7 ate the number 9"
Bethany: (lights now on) "OH!"
Me: "Daylin, did you not understand the joke before you told it?"
Daylin: "Um, no."
Me: (riotous laughter)
Daylin and Bethany: (slightly confused laughter)

Well at least we all ended laughing that time.