Understanding Main Effects

Life, thanks largely to interactions, can be complicated.
If there weren't interactions, how much simpler could life be?
To get an idea of how simple it could be, we have created a game that consists of a simple world in which there are no interactions.
Play the game below until you have

1. experienced examples of no main effects, main effects for one variable, and main effects for both variables; and
2. found that filling in the empty cell in a 2 by 2 table of means when there is no interaction is easier than playing tic-tac-toe.