India, July 26 -- Someone, in April 2026, is said to have walked out near the airfield at Charles de Gaulle and warmed a temperature sensor for a few minutes. A lighter, a hand warmer, perhaps the hair dryer of legend; the photograph that showed one turned out to have been generated by a machine.

The reading rose. On Polymarket, an online venue where strangers bet on the highest temperature in Paris, the tamperer collected about $20,000. The platform did not claw the money back. It merely changed which airport it would trust for the weather.

That small act contains the whole argument. A prediction market lets people buy a contract that pays one dollar if an event occurs and nothing if it does not; the price, held between zero and one, r...