Scientific American: Old cars that foul the air with dirty exhaust are a valuable commodity in Texas. If you own a clunker at least 10 years old and live near a major city like Houston, the state will give you a voucher worth more than the car’s value. Drive the car to a dealer, sign a few forms, hand over the voucher–worth up to $3,500–and drive off in a newer vehicle, no soot trailing you down the road. For the clunker, this swap is the last stop before being plundered for parts and scrap metal. But for …