Knock Knock

Moolah surprised Daddy and I at dinner by spontaneously delivering her first (to us) Knock Knock joke, followed by her second, third, and eventually approximatley 17th before we cut it off. Always the same set up, varying punchlines. She was very proud of it, although it needs a little work. It was something like:

Knock knock.
Who's there?
Mr. Potato Head.
Mr Potato Head who?
Mr. Potato Head I ate a shoe.

I searched the internet for a Mr. Potato Head joke to try to find the original punchline, but no luck. If you've got any good ones, put them in the comments. I'll be needing some soon!