For some reason, Joe and I were talking about ropes and pulleys. One thing lead to another, and soon we were in the back yard with some clothes line, pulleys, and a bucket. First we started with a bucket with a brick in it. Can you lift the brick? Yes. Can you lift two bricks... barely. How about three bricks? No way. Okay, over to the pulleys. A single pulley hung from a tree branch doesn't provide any mechanical advantage, but he had fun lifting the bucket up and down. Next, I added a second pulley for a 2:1 advantage and tied a loop in the rope so he could pull it across the lawn. Three bricks were now doable. Finally we, replaced the bricks with a particularly large zucchini. This proved to be way more fun.
There's no way that Joe, at three, is going to understand basic machines like this on an intellectual level. But if he develops some physical intuition by playing basic games like this, so much the better. Plus it's fun for everyone concerned.