Half of countries in the game are 'in' and half are 'out', according to the secret rule, i.e. the 'in' countries all have something in common that is not true of the 'out' countries. (3 'in' countries are given when you click Start to get you going.) The order that you click on the 'in' countries does not matter; just avoid the 'out' ones.
For each shot that you get in, another 'in' country will come back at you to help you make your next selection. Think carefully about your shot selection, or you will lose the rally! How long can you keep the rally going for? (The secret rule is revealed if you get to the end, then click ~~~CLICK TO FINISH~~~ to win the game.)