"I know a great one in the city, but he is very expensive: five hundred dollars for the first visit, and one hundred dollars for each one after that."
The woman went to the doctor's office and, trying to save a little money, cheerily announced, "I'm back!" thinking she could save the $500, and pay $100.
Not fooled for a second, the doctor quickly examined her and said, "Very good. Just continue the treatment I prescribed on your last visit." That will be $100.