An even more genteel version of paintball. Your goal is to get the red ball to the red square. The ball follows (more or less) the rule of gravity; when you click "Go" or press the spacebar, it drops straight down until it's diverted by one of the white lines.
Here's where you come in. You can add as many lines as you like to steer the ball to its target, making sure it doesn't drop to the bottom of the screen (or touch any of the other edges). The third level is a Rorschach test of sorts. Normal people will see a cuddly bunny. Dirty-minded people might see something else.
It isn't rocket science, but it's a nice way to kill a few minutes. There's music and sfx, which you can turn off via the "Options" menu. You can also design your own levels there.