Move the mouse to aim. Press any keyboard button to destroy nearby cats. There's one "original" cat that you can't destroy. You need to destroy the clones before they make more (like in one second). If you stop for a moment, they will clone too much...you'll lose the game if there's more than 100 clones. You'll win if you destroy 150 cats. Good luck :D
I got the idea when I discovered this online editor (with more possibilities than the old offline editor) and saw the clone thing xD