A man walked into a doctor's examining room.
"Put out your tongue," the doctor said.
The man complied, and the doctor took a quick look.
"Okay, you can put your tongue back now," the doctor said. "It's clear what's wrong with you. You need more exercise."
"But, doctor," the man began, "I don't think—"
"Don't tell me what you think," the doctor interrupted. "I am the doctor, not you. I know what you need. I see hundreds of people like you. None of them get any exercise. They sit in offices all day and in front of the television in the evening. What you need is to walk briskly for at least 20 minutes a day."
"Doctor, you don't understand," the patient said. "I—"
"I don't want to hear any excuses," the doctor cut in. "You must find time for exercise. If you don't, you will gain weight and have health problems when you're older."
"But I walk every day," the patient insisted.
"Oh, yes, and I know what kind of walking that is," the doctor replied dismissively. "You walk a few feet from your house to the train station, a few more feet from the station to your office, and a few more feet to a restaurant for lunch and back. That's not real walking. I'm talking about a proper walk in the park for twenty minutes every day."
"Please listen to me, doctor!" the patient shouted, growing angry with the doctor who thought he knew everything.
"I'm a mailman," the patient continued, "and I walk for seven hours every day."
For a moment, the doctor was silent. Then he said quietly, "Put your tongue out again, will you?"