Each player has 10 counters - either all red or all blue. Players take turns to place their counters on this board:

Before playing, a player can declare that a triangle with counters of one colour at every vertex has been formed and if correct that player becomes the winner.

So it is possible to make a triangle, not have your opponent spot what you have done, and then declare your triangle on your next turn ... and so become the winner.

The game can, of course, be played collaboratively. In which case, players work together to see how many counters of 2 colours they can place on the diagram without forming a triangle in either colour.