Play Dots and Boxes and learn who controls the chain

Draw one edge between neighbouring dots. Close a box to score and keep the turn. Boxline runs offline, needs no account, and explains the third side and long-chain control.

Method source: public-domain Dots and Boxes rules. Approved archive: daily-v2 with exact validation at 8-12 remaining edges; general exact solving is limited to 12 or fewer remaining edges.