stop me if you heard this one before

... a man walks into a doctor's office and says:
"Doctor, you have to help me. I keep thinking I'm a moth"
"Well, my good man", says the doctor, "you should see a psychiatrist. His office is just down the corridor."
" I know", says the man, "but in your office the lights were on."

Bertstravels
heard this one a long time ago, but is still laughing.