In 1960, Denys Fisher (1918-2002), a mostly self-taught engineer in the UK, was developing machines to draw patterns. While prototyping with Meccano—a model construction kit similar to Erector in the US—Fisher noted something in the design of a British pound note. "He thought its intricate patterns were made by a simple gearing mechanism," according to a BBC TV report below. "There and then, he decided to make a pattern drawing toy based on that principle."
While gear-based drawing toys had been available since the early 20th century, Fisher's device has the magic that made it a huge hit. Here's the story of the Spirograph:
(via The Kid Should See This)