A man goes to the psychiatrist and says, "Doctor, my sister thinks she's a dog."
"How long has she thought she was a dog?" the psychiatrist asks.
"Oh, ever since she was a puppy."
What’s So Funny?
When the man tells the doctor his sister believes she's a dog, we believe he and the doctor are on the same page. We suddenly realize that the man may be part of the delusion when he claims she has believed this since she was a puppy (young dog), not a young girl.