It just arrived safely this very morning, my Newton's cradle, which I bought like a month ago on eBay. It had been a while I wanted to have it on my desk. So one day, I browsed to see if there was any online shop selling the stuff. And there were a lot!
I stumbled upon a very good offer on eBay for a small design of Newton's cradle five-ball pendulum, and it was free shipping. So, within a minute the colliding balls, which was named after the English physicist Sir Isaac Newton, was already on my shopping chart.
I always wonder how this simple design, five small balls suspended by thin threads attached to two horizontal bars, works. I keep pulling one or two or three balls then release them and watch them rock back and forth.