A salesman, engineer, and a technician are driving just outside of town when they get a flat tire. The three get out of the car and scratch their heads.

The salesman says, "Maybe I should walk into town and get us a new tire. I know that I can
bargain with the man at the parts store and get us a great deal."

The engineer stops him, saying, "No, before you do that, we'll have to do some computations; figuring the grade of the road, the asphalt temperture, and the average rate of speed we will be travelling, to know what kind of tire you should buy."

The technician just laughs and shakes his head. . .

. . . "No, no, no! What's wrong with you guy's? We have a spare tire right in the trunk -- now all we have to do is start swapping tires until we find the flat one."