As a teacher, I love analogies. They connect what students know with what is being taught. They condense a web of ideas into a simplified picture of the essential elements.
I came across an analogy for Free versus non-FREE software on Italo Vignoli’s blog today. The blog is in Italian which Google translates passibly but the analogy is an image of people under an umbrella, a dependence on some supplier of non-free software, and a bowl, filled with people sharing. The idea is that the umbrella works and keeps off the rain, but one has to keep paying the holder of the umbrella or one gets wet whereas the sharing can go on indefinitely. I would choose a different model. Instead of an umbrella, the dependency should be a spider with users in hand, waiting to be released from lock-in or devoured.