polishing the brick
The great master Mat-su, as a youth, was a fanatic about sitting in meditation for many hours at a time. One day, his patriarch's disciple Huai-jang asked him what on earth he hoped to attain by this compulsive cross-legged sitting.
"Buddhahood," said Mat-su.
Thereupon Huai-jang sat down, took a brick, and started to polish it assiduously. Mat-su looked at him, perplexed, and asked what he was doing.
"Oh," said Huai-jang, "I am making a mirror out of my brick."
"You can polish it till doomsday," scoffed Mat-su, "you'll never make a mirror out of a brick!"
"Aha!" smiled Huai-jang. "Maybe you are beginning to understand that you can sit until doomsday, it won't make you into a Buddha."