Have you ever had the experience of having to drive a considerable distance with an empty gas tank? You knew that you were running low on fuel, but you were late for an appointment, in a hurry to get somewhere, and you kept thinking, "Yeah, I've got enough to get there," but you were unconvinced. You kept glancing at the fuel gauge, then your watch. The fuel gauge was slightly above empty, then the red warning light came on, then your fuel gauge rested solidly on the empty mark. Your stomach grew tighter and tighter.