Richard James was on Naval duty in a ship one day when he saw a torsion spring come loose and fall to the floor in front of him. He was intrigued by the way it flopped around on the floor, and remembered it well enough that when he returned home, he was able, with his wife, Betty, to construct a tight coil of a ribbon of steel. He fine-tuned it to be able to walk down stairs. This would be known as the Slinky, because of how it slinked along.
Although first Slinkys were made of metal, they now are available in many different colors of plastic as well. They have been in continuous production since 1945.