The Slinky was invented by accident by a naval engineer trying to stabilise ship instruments.
In 1943, Richard James knocked a spring off a shelf and watched it walk down onto a pile of books. His wife Betty named it Slinky, and the first 400 sold out in 90 minutes at a Philadelphia department store.
Why is this true?
A Slinky walks down stairs because the coils transfer energy along the spring faster than the whole spring falls. More than 300 million have been sold, and the original design has barely changed.
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More about inventions
Most inventions arrive by accident, in the wrong order, or decades before anyone finds a use for them. Microwaves, sticky notes, safety glass and vulcanised rubber all started as mistakes or side effects. The facts here trace how the thing actually came about, note who noticed the accident and why it mattered, and point at a source so you can follow the full story if it hooks you.