Once you've seen a box as a container, you struggle to see it as anything else

Karl Duncker's 1945 candle problem asks people to fix a lit candle to a wall using only a candle, a box of thumbtacks and a box of matches, without dripping wax on the table below. The trick is emptying the tack box and tacking it to the wall as a candle holder. Only 43% solved it when the box arrived full of tacks — but everyone solved it once the box was presented empty.

— Karl Duncker, On Problem Solving — Psychological Monographs, 1945