This riddle looks like a question about a bridge's weight limit, but it is really a question about the exact moment at which you look at the scene.
The Truck, the Bridge, and the Bird
Riddle statement
A truck enters a bridge weighing exactly the maximum allowed weight.
The bridge is 1 kilometer long.
The truck consumes 1 kilogram of fuel per kilometer.
Exactly halfway across the bridge, a 100-gram bird lands on the truck.
Does the bridge collapse?
Show solution
Solution
No, the bridge does not collapse.
When the truck enters the bridge, it weighs exactly the maximum allowed weight.
But when it reaches the middle of the bridge, it has already traveled:
kilometers.
Since it consumes 1 kilogram of fuel per kilometer, in half a kilometer it has consumed:
kilograms of fuel.
That means the truck weighs half a kilogram less than when it entered.
Then a 100-gram bird lands on it, that is:
kilograms.
The weight added by the bird is less than the weight lost in fuel.
The truck is still:
kilograms below the maximum allowed weight.
Answer: no, it does not collapse. The truck had already lost more weight in fuel than the bird adds.