Draughts

Twelve men each on the dark squares, moving one diagonal step forward at a time. Jump an enemy piece to the empty square beyond and it comes off the board. Reach the far row and your man is crowned a king, which can move and jump backwards as well as forwards.

One rule turns that into a game: you are not allowed to decline a capture. If a jump is available anywhere on the board, a jump is what you must play. That means a piece you leave in the way is not a mistake, it is an instruction — and setting up a position where your opponent is forced to take the wrong thing is how draughts is actually won. Two-for-one trades, a man offered to open a line, a run of three jumps in a single turn: all of it comes out of that one rule.

These are the English rules, on eight squares by eight, which is what a British or American player means by draughts or checkers. Kings move one square, not the whole diagonal — that is the international game, and it is a different one. Three strengths of opponent: easy looks one move ahead, medium four, and hard seven, which is enough to set a trap and wait for you to walk into it.