. . . Revolving doors are not only more fun than their swinging alternatives, they're also more energy efficient.

"When you open a door, a lot of air goes in and out, and a revolving door helps with keeping that more stable," said Rini Paiva, vice president for selection and recognition at the National Inventors Hall of Fame, which has a museum in Alexandria, Va. "It's a very convenient way to move people in and out of the building without having to keep a door open continuously."

That means more heated air can be kept inside during the winter, and cooled air during the summer, reducing energy use.

But when students at M.I.T. studied foot-traffic patterns at a campus building in 2006, they found that only 23 percent of people chose the revolving door over nearby swinging doors. (Not everyone can use revolving doors; they're usually inaccessible to people in wheelchairs, for example.)

The students calculated that if everyone used the building's revolving door, it would save 14.6 tons of carbon emissions annually — the amount generated by heating five households. Multiply that across all the revolving doors in the world, and the energy savings could be significant.