One matchstick turns 731 into -131: what matchstick puzzles reveal about self-imposed rules
aakashgupta · x · 2026-10-05
A matchstick puzzle, elegantly deconstructed: pull the top stick off the 7 and lay it flat in front of the number — the 7 becomes a 1, the moved stick becomes a minus sign, and 731 turns into -131, beating it by 862.
- Most people miss it because they silently assume the answer must be a positive three-digit number — a rule the puzzle never stated.
- Cognitive scientists use exactly these puzzles: in 1999, Günther Knoblich and Stellan Ohlsson showed that with false equations like IV = III + III, people solve numeral changes quickly but hit a wall when the fix touches the math itself (e.g., turning + into =) — the classic unchanging-elements finding.
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