There is an old mile-long rope bridge in South America with a 4-ton weight limit. In this case, even a grain of salt more than 4 tons would cause the bridge to fail. That's why there's an ultra-precise weigh station on each end. Now, a farmer drives up in his truck which is completely packed with live pigs and dead chickens. As he approaches the bridge, he is asked by the bridge authority to pull over at the weigh station. They weigh the truck and find that it weighs exactly 4 tons, right on the nose. So they say the farmer can cross, but only once they've cleared away the rest of the traffic.
Well, they cleared the bridge, and the farmer started across. Now, as he was crossing, a sparrow started to circle around the truck, and when the farmer was exactly halfway across the bridge, the sparrow landed right on the cab of the truck. About a minute later, the truck arrives safely on the other side of the bridge, and the bridge suffered no ill effects.
How is this possible?
Hint 1: The farmer didn't jettison himself or anything else from the truck.
