How to Solve It (Princeton Science Library)

It is often said that to teach any subject well, one has to understand it “at least as well as one’s students do.”
It is a paradoxical truth that to teach mathematics well, one must also know how to misunderstand it at least to the extent one’s students do

What
is the unknown?

Solving problems is a practical skill like, let us say, swimming. We acquire any practical skill by imitation and practice.