Paper and pencil
Number of players:
A smallish number of crosses ( + ) are drawn.
Extend one arm of a cross by a curve which ends at another arm of the same or a different cross. Now add a new cross-bar at some place along this new curve. Curves must not cross or touch other curves or crosses.
If your opponent has no valid move.
An example of the first few moves of a 2-cross game:
Variants: See Sprouts