In the late 1920s, Lithuanian psychologist Bluma Zeigarnik was having coffee in a Vienna cafe when she noticed something unusual about her waiter. He could recall in extraordinary detail the specific orders of every table he was currently serving. But when asked about tables he had already served and whose bills had been paid, his memory was strikingly poor. He remembered what was unfinished with precision. He forgot what was complete almost immediately. Zeigarnik recognized this as potentially significant and designed a series of experiments to investigate it systematically. Her results confirmed what she had observed: people remember uncompleted or...