Interesting Puzzles ...... 1 a day... Number 2

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?
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Doctor Plumber

A pipe burst in a doctor's house. He called a plumber. The plumber arrived, unpacked his tools, did mysterious plumber-type things for a while, and handed the doctor a bill for $600. The doctor exclaimed, "This is ridiculous! I don't even make that much as a doctor!." The plumber quietly answered, "Neither did I when I was a doctor."