A student was trying to cure her bad habit of biting fingernails. She was an excellent practitioner of sitting meditation and quickly surpassed her roshi in all but the comprehension of apparent contradictions. Her roshi, an old, fat woman, advised her on the issue of nail-biting to simply have more self-discipline.
The student looked at her fat roshi.
"The self-discipline I've got should be enough," she said.
"Everyone has some self-discipline. It is like breathing." The roshi nodded to herself. "That lets you know how important it is. Everyone breathes well enough when doing things to which they have become accustomed. For instance, I'll bet you breathe quite well even after a long run."
"Of course," said the student.
"That wouldn't be true for me," said the roshi. "I'm not used to running. But I am accustomed to self-denial and self-discipline. I wonder if you're as good as you think. Are you thirsty?"
"Not really. A little." The student shrugged.
"We have an hour lesson." The instructor set her bottle of water in front of her student. "Sit next to this water for an hour. But don't drink any."
Before the hour was finished, the student grabbed the bottle, uncapped it, and took a drink.
"The time is not yet," chided the roshi.
"Isn't it?" The student looked at the nearby clock. Her shoulders slumped. "I thought it was. Anyway, that was kind of hard, having the water sit here in front of me. It made me thirsty right away."
"You're really out of shape," said the roshi.