Duncker's candle problem exposed a blind spot in how we see everyday objects

Karl Duncker's 1945 experiment asked people to mount a lit candle on a wall using only a box of tacks, a candle, and matches. Most never think to empty the tack box and use it as a platform, because once an object is categorized by its usual function, the mind resists repurposing it. Duncker called this functional fixedness.

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