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?

 

[toggle title="What's the answer?"]Since the bridge is 4 kilometers long, the halfway point would be 2 kilometers. The 18-wheeler would have used much more than 30g of fuel to drive 2 kilometers.[/toggle]

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