in reply to Lexical scoping like a fox
Touching on the subject of recursion is important as well. It's easy enough to read this as if there were only one set of variables for a block, and each time you enter the block you get the same set over again, which isn't the case.
You don't necesarily have to go into detail on either of these, but it's important to make sure you don't give folks a mistaken impression that'll get in their way when they hit the more complex stuff later.
Other than that, this is pretty good.