Slinky  

Naval engineer Richard James inadvertently invented the stair-descending spring during a failed attempt at making stabilizers for ships' instruments during World War II. After he sold a bunch to a department store in Philadelphia in 1945, demand became so great that James started mass-producing them there. Production eventually moved to Hollidayville, Pa., where  more than 300 million have been made to date. It's still fairly cheap, too, with the average Slinky selling for roughly $5.  

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