I seem to remember reading a definition for a word that conveyed the idea of "an instance of doing something that prepares one for something else". Something like a lemma but not restricted to mathematics.
Did that describe a word you know? I'm 95% sure there's actually a simple noun that specifically denotes something that you use to get ready for the more challenging version of the same thing.
My question isn't limited to what I'm writing right now, I'm just curious - but here is a little more context: I'm introducing the reader to game theory. I take her through two games. One prepares her for the other. It isn't practice, it's just getting familiar with it. I'd liken it to the climb to Everest base camp. In fact, you could use that, "The climb to Everest base camp is a ___ for the climb to the summit."