Tic-tac-toe

Everybody learns this game at about six and stops thinking about it at about seven, which is a shame, because there is exactly one idea in it worth knowing. It is the fork: a move that leaves you one square from finishing two different lines at once. Your opponent can block one of them. That is how games of noughts and crosses are won, and it is the only way they are won against anybody paying attention.

Three opponents, and they are three different players rather than three speeds of one. Easy finishes a line it has already built and blocks nothing, which is the opponent a child beats. Medium plays the rule everybody learns first — win if you can, block if you must — and loses to a fork. Hard searches every position on the board and cannot be beaten.

That last sentence is a statement about the game, not a boast about the computer. Noughts and crosses is solved: with best play from both sides it is a draw, every time. A draw against the hard opponent is the best result there is, and getting one takes more than you would think.