WHY does interleaving beat blocking?

It forces you to figure out WHICH kind of problem you face and WHICH strategy applies — a discrimination skill blocking never trains — and it also spaces each type out.

In a blocked set, the block itself gives away the answer type: you already know it's a sphere, so you only practice executing the sphere formula, never choosing it. Real tests and real life don't announce the problem type. Interleaving trains exactly the missing skill — recognizing what you're looking at and selecting the right approach — which cognitive scientists call discrimination. As a bonus, mixing types automatically spaces your return to each one, layering the spacing effect on top. So interleaving delivers two desirable difficulties in one.