Connect nine dots in a square grid using only four straight lines
Nine dots are arranged in three rows of three, like a tic-tac-toe grid with no lines drawn. Using one continuous stroke of exactly four straight lines, can you pass through every dot, without lifting your pen or retracing a line?
Reveal the answer
Yes — but only if your lines extend beyond the square shape the dots seem to imply. Draw a diagonal through three dots and keep going well past the grid's edge, then angle back through the remaining dots; no four-line solution stays neatly boxed inside the dots. Sam Loyd published it as the 'Columbus Egg Puzzle' in The Strand Magazine in 1907 and later in his 1914 Cyclopedia of Puzzles; it became the classic illustration behind the phrase 'thinking outside the box.'
— Sam Loyd, Sam Loyd's Cyclopedia of Puzzles — 1914 (puzzle first published in The Strand Magazine, 1907)
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