mentally illness =P

A doctor wanted to see if any of his mental patients have recovered from their
illness, so he had one room prepared with a fake door painted on the wall.
Then he let in all the patients and observed their actions. Almost immediately,
every patient except for one went to claw at the fake door, trying to open it.
The only one who who did not join the others simply sat at a corner and watched
them calmly, thus the doctor thought that particular patient had recovered. So,
the doctor told him that he was discharged from the hospital and free to go.
Hearing this happy news, the patient decided to share his secret with the doctor.
He motioned the doctor closer and whispered into his ear, "Psst, so I tell you,
but don't tell the others, I have the key to that door..."