This is a ball mill used for refining materials into a fine powder. [Jpoopdog] built it in two parts, a base and the tumbler chamber. The base itself is build using LEGO wheels as rollers.

The World’s Coolest Keystroke, built by [Audiobody], is made from a combination of tennis balls, Lego bricks, servos, and switches. When a tennis ball is lifted up, a Lego arm is actuated.