It's Slinky! Even though I could never get my Slinky to go down the stairs in the 70s like they did in the commercials, I was hooked on the simple yet genius design; one material, a strong geometry, so obvious once you see it.
The idea was sparked in 1943 by Richard James who was working with springs as an naval engineer on a battleship. As the story goes a spring fell on the ground but kept moving, thus the idea for a toy. His wife, Betty, is credited with naming it Slinky - a Swedish word for traespiral, sleek or sinuous. In 1945 it debuted at Gimbel's Department Store (400 were sold during the 90 minute demonstration) in Philadelphia during the Christmas season and then went onto the 1946 American Toy Fair.
Slinky's are still made today with the original equipment designed and engineered by James; each one is made with 80 feet of wire. Over a quarter billion (billion!) have been sold. How's that for an "Aha" moment!