Some people hunt for the answer. Others hunt for why it’s the answer. Only the second group can handle a curveball.

An answer is a single point. The reasoning behind it is a map, and maps keep working when the terrain shifts. People who collect only answers do fine until a question arrives in an unfamiliar costume, at which point they have nothing to walk with. The difference is invisible in easy conditions and total in hard ones.

After you get something right, spend one more minute asking why it is right and what would have to change for it to be wrong. Try a slightly altered version of the same problem. If you can still solve it, the understanding is real.