A not-so-bright couple found a place that rented rowboats on a nearby lake. They paid the fee and were assigned one of the boats, which they took out to fish. The man and woman took turns rowing as they moved about the lake and tried casting at spot after spot. For a long time the fish just didn't seem to be biting. But at last they hit on a wonderful place by a patch of lily pads, where they began reeling in fish after fish.
As he pulled the hook from the mouth of a particularly nice bass, the man mused, "Gee, I wish we could mark this spot so we can remember where to come back to next weekend." The woman thought a bit, then smiled and pulled an ink pen out of her pocket, and made a big X on the side of the boat. The man shook his head and said, "I doubt that will work. Stop and think! What if we don't get the same boat next time??"