Why Not Quit?

The new patient was airing his woes to an understanding doctor.

"After the first, I'm tired, Doc. After the second, my chest aches and I start getting pains in my legs. After the third, I feel like fainting and it takes half-an-hour for my heart and respiration to return to normal."

"Why don't you quit after the first?" inquired the doctor.

"How can I do that, Doc?" said the patient. "I live on the third."