Nathan and Mary were both patients in a mental hospital.
One day while they were walking past the hospital swimming pool, Nathan suddenly jumped into the deep end. He sank to the bottom of the pool and stayed there. Mary promptly jumped in to save him. She swam to the bottom and pulled Nathan out.
When the Head Nurse became aware of Mary's heroic act she immediately ordered her to be discharged from the hospital, as she now considered her to be mentally stable.
When she went to tell Mary the news she said, "Mary, I have good news and bad news. The good news is you're being discharged; since you were able to rationally respond to a crisis by jumping in and saving the life of another patient, I have concluded that your act displays sound mindedness. The bad news is, Nathan, the patient you saved, hung himself with his bathrobe belt in the bathroom. I am so sorry, but he's dead."
Mary replied "He didn't hang himself. I put him there to dry. How soon can I go home?"