https://onlywei.github.io/explain-git-with-d3/
This thing is amazing: it uses SVG to visualize exactly what's happening to the test repo when you run various commands, including the stuff that is normally invisible, which can be key to realizing how powerful git truly is. I played with it for about an hour and clarified a bunch of things I half-understood.
It won't do everything, but it will do a ton of the things that I've scratched my head about over the years.