KEEP YOUR EYE UPON THE SPARROW

Congratulations to Cadet Samantha "Little Fish" Migliori for correctly noting that by the time the truck reached the center of the bridge and the sparrow alighted, it (the truck, not the sparrow!) would have burned more than 30 grams of fuel. 

Here's a new cadet challenge:

An 18-wheeler is crossing a 4 kilometer bridge that can only support 10,000 kilograms and that's exactly how much the rig weighs. Halfway across the bridge a 30 gram sparrow lands on the cab, but the bridge doesn't collapse. Why not?

This challenge will end at 0730 Monday, 24 September.  The winner will receive a delicious soft drink.