For years, this has been my favorite joke:
you: Who's there?
me: Interrupting cow.
Kinda hard to get that across in writing, but trust me. It's funny. I did, however, hear a one today that might just be my new favorite:
A lonely guy goes to a pet shop to get himself a new pet. The pet shop owner says, "Have I got a pet for you! I have this centipede who talks and sings opera. He'll be perfect for you." The guy is dubious, but decides to take a look. Sure enough, the centipede comes out of his little house, carries on an intelligent conversation, and sings a glorious aria. "Sold!" says the guy.
After a few days of scintillating conversation and beautiful singing, the guy decides he has to show off his amazing new pet. He knocks on the roof of the centipede's little house and says, "C'mon, let's go down to the bar. You can meet the fellows and show off your voice. It'll be great!" No response.
The guy tries again. He knocks on the roof, encourages the centipede to come on, already. No response.
He tries a third time, this time getting a little snippy. "What's the matter? Are you shy or something? Come ON!"
Finally, he hears a voice from inside the house."Keep your shirt on! I heard you the first time; I'm putting on my shoes!"