A sliding stone from Sheffield.
When I was nine years old we used to 'play on the railings' in our break times at Gleadless middle school in Sheffield. These thin metal railings swept down the side of twenty odd steps that led to the netball court. Our game (which of course, was very popular) was to try to slide a stone from the top of the railing to the bottom, without the stone falling off in-between. This was surprisingly hard to do.
One day I found this small stone; a random piece of sandstone, aerodynamically designed for rail sliding. I spent quite a few hours grinding it into shape against another stone, just to perfect the almost perfect lines. Then, one dinner time it was ready for testing. In front of my friends and fellow stone sliders, I let my stone slide. 

From the top to the bottom, it sped along the rail without falling off! 
In a moment of celebration and amid cheers from my friends (maybe) I ran down the steps to retrieve my prized rock. Jumping the last few to get my stone, I landed badly and snapped my ankle bone in half. After a visit to the hospital, I had seven weeks in a cast and a lot of time off school. 

I never slid my stone again.