While driving down the road one day, a motorist sees a truck (closed van) coming up behind him rapidly. The van passes him, but a short time later he sees it parked along the road, and the driver is out beating the side of the truck with a tire iron. He goes on, and soon he sees the truck rapidly approaching again. This scenario repeats several times. His curiosity perked, the next time he sees the truck stopped, he stops also, and queries the driver. "Well, I have 20 tons of canaries in here, and this is a 10 ton van, so I am trying to keep them flying"! What is wrong with the physics?? If the van was a cage instead of closed wat diffrence would it make?