By Daniel T. Willingham

Question: Memory is mysterious. You may lose a memory created 15 seconds earlier, such as when you find yourself standing in your kitchen trying to remember what you came there to fetch. Other seemingly trivial memories (for example, advertisements) may last a lifetime. What makes something stick in memory, and what is likely to slip away?
Answer: We can’t store everything we experience in memory. Too much happens. So what should the memory system tuck away? How can the memory system know what you’ll need to remember later? Your memory system lays its bets this way: if you think about something carefully, you’ll probably have to think about it again, so it should be stored. Thus your memory is not a product of what you want to remember or what you try to remember; it’s a product of what you think about.

A teacher once told me that for a fourth-grade unit on the Underground Railroad he had his students bake biscuits, because this was a staple food for enslaved people seeking escape. He asked what I thought about the assignment. I pointed out that his students probably thought for 40 seconds about the relationship of biscuits to the Underground Railroad, and for 40 minutes about measuring flour, mixing shortening, and so on. Whatever students think about is what they will remember.

The cognitive principle that guides this article is memory is the residue of thought. To teach well, consider what an assignment will actually make students think about (not what you hope they will think about), because that is what they will remember.