The Slinky was one long piece of curly metal that, according to the adverts, you would make walk down stairs. You set it on the top step, flipped the top section of the Slinky onto the step below and momentum pulled it down onto itself and flipped down onto the next step down.
Theoretically, if you put a Slinky on an escalator, it could walk for eternity! Just think about THAT for a moment! Perpetual Slinky!
In reality most people tried to walk it down the stairs a couple of times and gave up. They would spend the rest of the time holding both ends while bouncing the mid section up and down, before tangling it and binning it.
I never straightened a slinky out but I remember reading somewhere years ago that if you stretched one until it was flat it would be 67 feet long. If anyone ever asks you what the biggest toy you owned was, bear this in mind – your Slinky was over 20 metres!
The modern day Slinky, and the generic plastic ones, are much shorter than their 70s and 80s originals, and so will not descend stairs anywhere near as easily. The cheaper ones are better designed for holding and boinging.