Why an unpaid restaurant bill sticks in your head

Psychologist Bluma Zeigarnik noticed a waiter could recall every detail of unpaid orders but drew a blank once the bill was settled. Her 1927 experiments confirmed it: people remember interrupted, unfinished tasks far better than ones they've completed. It's why a half-written email nags at you more than a sent one.

— Bluma Zeigarnik, On Finished and Unfinished Tasks (Über das Behalten von erledigten und unerledigten Handlungen) — Psychologische Forschung, vol. 9, 1927