This is way better than the standard “Waiter, there’s a fly in my soup” joke. Here it goes:
A guy runs in to a psychiatrist’s office and says, “Doc, I need help. My brother thinks he’s a chicken.”
The doctor says, “Why didn’t you send him?”
“Because we need the eggs,” the guy replies.
I think I first heard this in a Woody Allen movie, but I disremember.