Mouse to aim, click to shoot. Don't let the strangely horizontal path of the asteroids take them past your cannon, or you lose. Hold down the mouse button to fire continually. I think that's about it. There is no secret way to win. Don't go looking for one or anything. I might be saying this to pique your curiosity and get you to play again to look for one, or I might be saying it as a hint that there really is a secret ending. What do you think?
Woah, wait, how come I didn't see the comments until a month later?
