Circle Sumo
In this game, circles represent sumo wrestlers (or open sets). You can drag your circle around to move the circle (or change the "set"), and push up against the opponent's circle to move them.
15 matches - You win a match if you knock your opponent out of the white circle. While playing, think about how this relates the white circle's center, your wrestler circle's center, and their radii. (This concept shows up in geometry, topology, and computer progamming.)