Knock, Knock. Kaboom!

Leo was trying to tell Katie a knock-knock joke. It went something like this:

Leo: Knock, Knock.

Katie: Who's there?

Leo: A tree.

Katie: Kaboom!

Katie, Leo & Max: Laughing.

Leo: Knock, Knock.

Katie: Who's there?

Leo: A house.

Katie: Kaboom!

Katie, Leo & Max: Laughing.

I asked Max, "What's so funny?" Max said, "Whenever someone knocks at Katie's door she blows them up!"