The Slinky, that spring thing that walks down stairs, was invented in Philadelphia nearly 60 years ago, but it still has surprises up its helical sleeve.

And we're not talking new novelties à la the Slinky Dog or Slinky eyeballs.

A Slinky amazingly "walks" on a treadmill for minutes, flopping and flipping along, even self-correcting its course, on a YouTube video that has been seen more than 3.3 million times in just two months.

Now comes some cool slo-mo of another freaky trick — how a Slinky seems to momentarily hang in mid-air as if it has some anti-gravity power.

It happens when the coil toy is held from the top and the bottom is let loose to hang. Then, when the top is let go, the top drops, as expected, but the bottom stays still — seemingly suspended by invisible string — until the whole toy becomes a bunched-up plummeting bundle.

In just two weeks, a new YouTube video of the mind-boggling phenomenon, has nearly a half-million views. "Awesome HD Slinky Slo-Mo" is not the first demonstration of the trick but this version has cooler slo-mo and more science talk.

"You're changing something at the top, and there's a finite time for that information to get to the bottom of the Slinky," explains associate professor Mike Wheatland, a solar astrophysicist at the Sydney Institute for Astronomy.

Think of it this way: The bottom doesn't "know" anything has changed above, and still "thinks" it's being held up, so it doesn't move.

In more concrete terms, the top's descent doesn't immediately change anything below, so the tension keeps pulling the bottom up until the whole thing collapses.

Wheatland points out that, as a whole, the Slinky actually is falling, since its center of gravity does drop at an accelerating rate.

Perhaps even more stunning is the same trick with a tennis ball tied to the bottom. The ball's weight makes no difference! It, too, hovers.

One wonders: If you hold the top and let the bottom fall, the bottom will bounce upward when it fully extends. What if you drop the top as the bottom is rising? Assuming the information idea is right, the bottom should keep rising, producing an even more gravity-defying effect.

Hmm. But would that be as impressive? Springs bounce up, no big deal. Stationary objects, that rattles the brain.

The Slinky was invented in 1943, after engineer Richard James, aboard a ship at the Philadelphia Navy Yard, saw a torsion spring fall off his desk and tumble end over end across the deck. The idea for a new toy sprang to mind, so to speak. His wife, Betty, chose the name, and the first ones were sold at Gimbel's in Philadelphia in 1945. It's been bouncing along ever since, selling a few hundred million over the years.