“Flow” seems particularly applicable to ZPD — too easy (no stress) -> no learning, and too hard (too much stress) -> no learning.

On the example of “heavy” — maybe the two distinct aspects of gravitational weight and inertia might be a good example?

The example of sky color is even more believable than the famous fail in machine learning where an MLP was trained to discriminate male from female faces, but all the images of one sex were offset from the bottom of the image by a different number of pixels than the other!

So — overall rating for the so-called “blog post”, actually a PhD dissertation chapter in disguise: 5.5 / 5.0 !!