Dave and Bo went elk hunting every year. The forest was so thick that they had to hire a helicopter to take them in and out. At the hunt's end, Dave and Bo called up the helicopter to come and get them and the six elks they had shot.
On arrival, the helicopter pilot looked over the catch. "I can only carry half of your catch," he said. "Six elk would be too heavy for the helicopter."
"Last year the helicopter carried six elk," replied Dave. "And it was the same type of helicopter as yours and the weather conditions were exactly the same."
The pilot succumbed to persuasion and took aboard the hunters and their six elk. The helicopter could not gain height and skimmed along the tops of the trees for a mile or so, and then crashed. Fortunately no-one was hurt.
"Do you know where we are?" asked Bo.
"Yes," replied Dave, "about a hundred yards from where we crashed last year."