The basic aim is to throw the ball / severed animal head that your mouse is holding into the balls of the same type that are freely bouncing around the screen. In this case, you can see that the mouse is holding a panda head, so throwing it into that group of four panda heads will cause all the panda heads to disappear. If it hits a non-matching ball, like the pink cat, the blue dog or the golden pig, it will stick to them and make the clump bigger.