Willis Carrier invents the world’s first mechanical air-conditioning unit for a Brooklyn printing plant, where the muggy air makes magazine pages wrinkle.

The invention not only solves that problem, but results in huge cultural and economic changes in hot and humid areas of the U.S.

Five businessmen from Two Harbors, Minnesota, start Minnesota Mining and Manufacturing Company (3M) to mine what they thought was corundum, a mineral ideal for making sandpaper and grinding wheels.