low floor, high ceiling, wide walls

Speaking of the design decisions behind Scratch, Mitch Resnick from MIT Media Lab tells in one of the course videos that "one of the things I learned from Seymour Papert is that he used to talk about developing technology with a low floor and a high ceiling, meaning it's easy to get started, the low floor, and you can do more and more sophisticated things over time, a high ceiling. We sometimes also talk about having wide walls, meaning that you can have many different pathways".
Nice metaphors :-)