I once caught an 11th-grader who snuck a cheat sheet into the final exam.

At first, he tried to shuffle it under some scratch paper. When I cornered him, he shifted tactics. "It's my page of equations," he told me. "Aren't we allowed a formula sheet? The physics teacher lets us." Nice try, but no dice. The principal and I rejected his alibi and hung a fat zero on his final exam. That dropped his precalculus grade down from a B+ to a D+. It lingered like a purple bruise on his college applications.

Looking back, I have to ask myself: Why didn't I allow a formula sheet? Cheat sheets aim to substitute for memorization, and I hate it when my students memorize things.

"What's the sine of π/2?" I asked my first-ever trigonometry class.

"One!" they replied in unison. "We learned that last year."

So I skipped ahead, later to realize that they didn't really know what "sine" even meant. They'd simply memorized that fact. To them, math wasn't a process of logical discovery and thoughtful exploration. It was a call-and-response game. Trigonometry was just a collection of non-rhyming lyrics to the lamest sing-along ever.

Some things are worth memorizing--addresses, PINs, your parents' birthdays. The sine of π/2 is not among them. It's a fact that matters only insofar as it connects to other ideas. To learn it in isolation is like learning the sentence "Hamlet kills Claudius" without the faintest idea of who either gentleman is--or, for what matter, of what "kill" means. Memorization is a frontage road: It runs parallel to the best parts of learning, never intersecting. It's a detour around all the action, a way of knowing without learning, of answering without understanding.