Dots and Boxes
On a grid of dots, two players take turns drawing one line between two adjacent dots. Whoever draws the fourth side of a small box claims it and immediately takes another turn. Once all lines are drawn, whoever claimed more boxes wins. Simple as the rules sound, why do skilled players spend most of the game avoiding completing boxes rather than chasing them?
Reveal the answer
Because drawing the third side of a box hands your opponent a free point — so strong players spend the early game making only 'safe' moves that don't create a third side, forcing the opponent to eventually run out of safe moves first and start handing over boxes. Expert play centers on 'double-crossing': sacrificing two boxes in a chain to force the opponent to open the next chain for you, often letting you claim far more boxes than you gave away. First published in the 19th century by French mathematician Édouard Lucas, the game is deceptively deep — determining optimal play is classified as PSPACE-complete, among the hardest classes of problems in computational complexity theory.
— Édouard Lucas, Dots and Boxes — 19th century, as 'la pipopipette'