If I ever get skin cancer, I’ll blame it on the time I burned myself so badly I thought my face was going to peel off. Hiking high up on Mount Rainier, in Washington, where the snow reflects light onto one’s face from all directions and the frigid air keeps skin numb to the injury, I put on SPF 100 at dawn and didn’t think about it again. The next morning I was so inflamed it hurt to smile.

To understand why people like me make such stupid and consequential mistakes—one in five of us will get skin cancer, despite most cases being preventable—last year a trio of dermatologists at Northwestern University asked its patients to take a quiz about sunscreen.