some comments, one should see a reduction in weight--yet nothing has left the box. Ridiculous on the face of it. It depends completely on how lucky the driver is. We can calculate how much his luck is running in or running out, by counting how many rabbits feet, four-leaf clovers, and horse shoes he is carrying. Assuming that one horse shoe weighs about 1kg, an amputated appendage of rabbit weighs about 0.1 kg, and a four-leaf clover weighs about 0.001 kg, his best investment would be to look around for the closest clover patch and stock up on mostly those, before heading over the bridge. The horse shoes should be thrown as far as possible over the bridge to the other side, before crossing after them. This assumes, of course, that he crosses his fingers, which would not actually add any appreciable mass to his cargo, yet would boost his fortune to a moderate degree, nevertheless. Thank goodness he is driving, so he doesn't have to worry about stepping on cracks! (Hey! If the question does not reflect a real-life situation in the SLIGHTEST degree, then the answer certainly doesn't have to be real either, for it to be truly correct within the initial fantasy la-la-land parameters). "Draw an imaginary box around the truck. On the outside of the box, when the birds are in flight according to some comments, one should see a reduction in weight--yet nothing has left the box. Ridiculous on the face of it." But this