Avoid everything red. Shoot anything that moves. WASD or Arrow Keys to move, mouse to aim and click to shoot.
I made this because I got bored in class one day. I forgot to make it so that dead squares disappeared, but then I thought It would be a cool mechanic if things you shot were still dangerous, so you have to avoid them while shooting the live ones. So, I gave the game a nice UI, added sounds, and now it's pretty goode.