I’m repairing a power drill (more on that later), and I found inside it a rather excellent ball bearing.
* Why is “ball bearing” a synonym for “steel ball”? A ball bearing is a bearing, i.e. a rotary fitting that supports a spinning shaft, that uses balls to reduce contact area. It isn’t a marble made of steel. I’m guilty of calling the balls “ball bearings”, but the more I think about it the more I wonder why we needed a more obscure and less accurate metonym for “steel ball”.