In his research paper titled "The Falling Slinky," Canadian physicist W. G. Unruh observed something strange about the venerable Lazy Spring: "The bottom stays at rest until a wave hits it from above." This high-speed video demonstrates just that, and it's pretty insane to watch.
In the image above, you can see the phenomenon at work. A fully extended Slinky at rest is released from the top, and its body remains seemingly rigid and tube-like while the coil begins to collapse downward. It's almost like the Slinky's body is simply hovering there, instead of falling all at once, while it waits for itself to compress.
The Slinky isn't actually hovering, though at the same time it kind of is. The force of gravity is still acting on the length of the spring, but the tension inherent to the design of the coil is pulling it in an upward direction. When released, for a very brief moment, these two opposite forces make the Slinky look like it's levitating.
"You're changing something at the top," Mike Wheatland, an associate professor at the Sydney Institute for Astronomy, explains in the video below, "and then there's a finite time for that information about the change to get to the bottom of the Slinky. According to Wheatland, this all happens over the course of about a third of a second to the length of the Slinky, and happens in other objects, too — even something like a solid rod — but far less visibly.
You can see it for yourself in the video below. Also, mathematical minds can find W. G. Unruh's succinct paper on the matter here.