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.
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Update (1/18): Dragging balls outside the level should now be fixed. By popular request, the balls respawning are a little more intelligent—if you’re making a ball on the right side of the screen, they’ll respawn top left (and vice versa). Yay feedback!