A woman goes to see the psychiatrist, "Doctor, my brother thinks he's a chicken."

"Oh, my goodness!" says the psychiatrist. "How long has he thought he was a chicken?"

"Oh, about two years now," says the woman.

"What? Your brother has thought he was a chicken for two years, and you're only coming to see me now?"

"Well…," the woman shrugged, "We needed the eggs." 

Vocabulary

to shrug = to raise the shoulders to show a person doesn’t know what to do, to show uncertainty 

What's So Funny?

We assume that everyone thinks the brother is crazy to believe he's a chicken. (We do, the psychiatrist does, and the sister does.)  However, when the psychiatrist asks why the family took so long to bring the brother's case to medical attention, the answer is that the eggs were important to the family. This claim goes against the case that the brother is crazy because he seems to be producing eggs (just as a real chicken would do).