A Little Birdie Told Me...

This was taken from one of the most awesome websites in the world. I modified a few of the words, though.

Question: A truck that is transporting birds is weighed. Will there be a difference in its weight depending on whether or not the birds are in flight or standing on the truck?
Answer: There will not be a difference between the two situations. While flying, the birds' wings exert a downward force on the air in the truck which is exactly equal to the weight of the birds, and the air transmits this force onto the truck, and hence to the weighing mechanism.
The only way to cheat the weigh scale would be to have very proficiently trained birds that could fly to the top of the truck and, immediately before the weight is calculated, fold their wings and fall. If the weigh scale is read during the interval before the free-falling birds strike the floor of the truck, then it will register only the weight of the truck.