Knock Knock! Help Needed!

C is very into "Knock Knock" jokes these days, and my repertoire is becoming quite limited. I have pretty much run through my standard six to eight jokes I know and am seeking some new ones (clean enough for the preschool set please!). The sillier the better! If you have any, please feel free to leave them in the comments section or send them to my Hotmail email account as soon as possible. Joke telling is a popular way we pass the time in the car, especially on hot days when we have much "errand-running" to do (see previous post, conversation 3). Thanks for any help!