One day a doctor went to a store and bought a pair of shoes. Before he left the shoes counter, he asked the salesgirl: " How long will this pair of shoes last?"

"It depends. If you don t use it, the shoes will never wear out."

Several days later the salesgirl fell ill and went to a hospital. And the happened to be the customer she served. After the girl got the prescription from the doctor, she asked: "How soon will I get better with the medicine?"

"It depends. " The doctor answered, "If you don t use it, you will never get better.