Charles B. Brenner and Jeffrey M. Zacks in Scientific American:
The French poet Paul Valéry once said, “The purpose of psychology is to give us a completely different idea of the things we know best.” In that spirit, consider a situation many of us will find we know too well: You're sitting at your desk in your office at home. Digging for something under a stack of papers, you find a dirty coffee mug that’s been there so long it’s eligible for carbon dating. Better wash it. You pick up the mug, walk out the door of your office, and head toward the kitchen. By the time you get to the kitchen, though, you've forgotten why you stood up in the first place, and you wander back to your office, feeling a little confused—until you look down and see the cup.
So there's the thing we know best: The common and annoying experience of arriving somewhere only to realize you've forgotten what you went there to do. We all know why such forgetting happens: we didn’t pay enough attention, or too much time passed, or it just wasn’t important enough. But a “completely different” idea comes from a team of researchers at the University of Notre Dame. The first part of their paper’s title sums it up: “Walking through doorways causes forgetting.”