We just learned about the Axle.
Another part of machine work is a Bearing.
This is something that lets one part of a wheel spin around without moving another.
Like on a fidget spinner, you can hold the middle of it and the outside will spin around.
Or like a front bicycle wheel, the wheel will spin around the hole in the middle where the front handlebar is attached.
Usually this is done using ball bearings, which are tiny little balls covered in slimy grease.
On the outside is one metal cylinder, then a bunch of ball bearings, and then a smaller inside cylinder.
The outside cylinder can spin while the inside one is stopped, or the other way around.
(from: wikipedia - bearing (mechanical))
Kid Facts - Blast from the past: Attitude Control
