Every October, more than a hundred thousand spectators gather to watch brilliantly colored hot air balloons soar above New Mexico as part of the Albuquerque International Balloon Fiesta. This spectacle is made possible by the way air molecules—groups of atoms bonded together—inside the balloons behave when heated.
Before launch, workers use high-powered fans to fill each balloon with air. Once inflated, pilots light a burner at the mouth of each balloon, which produces a 3.7 meter (12 foot)-tall flame. The flame heats the air inside, causing the gas molecules to move faster and spread farther apart.
“The fast, hot molecules hit colder, sluggish molecules, transferring heat,” says Becky Thompson, a physicist at the American Physical Society. As more molecules inside the balloon heat up, the air becomes less dense than the air outside. That produces a force called buoyancy that lifts the balloon up, up, and away!