Hey, just look at this! It's a completely beautiful thing!
At each stage, you read a thing and then get a little toy to play with. The interactive vignettes do what games to do well — they allow you to feel exactly how a set of rules combine to produce unexpected results.
The content is very clear and smart, and I can't possibly explain it any better than they do. So go check it out!
I really love this interactive essay format. It looks like there are some more being collected at ExplorableExplanations.com, which I'm going to have to take a look at.