Mouse down to create a filler ball. Release the mouse to freeze it. You have a limited number of lives and balls each level. If you squash a bouncing ball, it will reappear at the top right corner.

In Filler, your goal is simple: fill 2/3 of the level. To create a filler ball, press down. It will grow until you release the mouse button, it hits another filler ball, or a bouncing ball runs into it.