Anything funny must be a knock-knock joke

So my kids really like the movie Atlantis.  There is one part, in particular, that they find really funny, and they run around quoting it to each other all the time to make themselves laugh until they are blue in the face.

The funny bit, it goes something like this:

Trustee #1:  "I swear that Thatch gets crazier every year."

Trustee #2:  "If I ever hear the word Atlantis again, I'll step in front of a bus."

Trustee #1:  "I'll push you!"

Trustee #2:  "Good lord!  There he is!"

That's how Connor recites the bit, and then Lex and he will laugh and laugh.  Sometimes they'll ask me to recite it for them.  When I do, they fall on the floor.

Lex knows it, too.  Only he always starts is like this:

"Knock, knock.  If I ever hear the word Atlantis again, I'll step in front of a bus.  I'll push you!  Oh good lord!  There he is!" 

Because if something is funny, then it must be a joke.  And if it is a joke, then it must start with "Knock, knock."  I mean, right?