Res ipsa loquitur

Oh, yes, I hear this daily. The funny thing is, it's not significantly more funny when told right than when told wrong.

The only knock-knock joke that still makes me laugh is the interrupting cow joke, which probably doesn't work too well in writing, but here goes:

Knock-knock.
Who's there?
An interrupting cow.
An interrup--
[interrupting] Moo!