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.