Tell me a good "Knock Knock" joke!!!

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Let me first start out by saying that this is NOT a game that I am posting just to get responses. I am not promising a best response to anybody unless I just truly think it is the best response! Now, down to business... I have a 4 year old who just learned the concept of a knock knock joke. The problem is that he only knows two of them, and they are getting redundant. In fact, I am SICK of them, but for the life of me I can't think of any new ones to teach him so we will stop with the two he knows. So, please, if you know any good ones save my sanity and post em here! Thanks!