Red Vertex: Vertex on fire
Green Vertex: Vertex protected by a fireman
Left click a non-burning, non-protected vertex to protect it with a fireman.
Right click a non-burning, non-protected vertex to set it on fire.
You can hold down mouse buttons to protect or burn multiple vertices.
Press the right arrow key to make the fire spread automatically.
Press the left arrow key to undo events. An event is a turn of fire spreading (with at least one new fire), or the placement of a new fire or fireman on the grid. You can undo multiple events by pressing left repeatedly.
Press R to reset the grid.