Does anyone really need to be told what a Slinky® is?? Okay, it is a very slack spring, 2-7/8" dia x 2-1/2" long, compressed. It will stretch out to about 5 or 6 feet long without deforming. That's the boring part. The interesting part is that it "follows" itself. Slosh it from hand to hand. Put in on the top step of stairs, flip the end down a step, and watch it walk down stairs, or down an incline (they can do it alone, but we recommend them in pairs). Believe us, no childhood (or adulthood) is complete without an original steel Slinky®.