What is your favourite Knock Knock joke

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My five year old son has recently discovered knock knock jokes. The problem is he only knows a couple of them, and he says them all the time. Please let me know what your favourite knock knock joke is so I can teach my kid some new ones. I'll start: Knock knock? Who's there? Lettuce. Lettuce who? Lettuce in it's cold outside!!! That kind of an idea.