sgt-puzzles/html/blackbox.html

Black Box
<p>
Determine where the hidden balls are in the box, by observing the
behaviour of light beams fired into the box from the sides.
<p>
Click in a square around the edge of the box to send a beam into the
box. Possible results are 'H' (the beam hit a ball dead-on and
stopped), 'R' (the beam was either reflected back the way it came or
there was a ball just to one side of its entry point) or a number
appearing in two squares (indicating that the beam entered one of
those squares and emerged from the other).
<p>
Click in the middle of the box to place your guessed ball positions.
When you have placed enough, a green button will appear in the top
left; click that to indicate that you think you have the answer.
You can also right-click to mark squares as definitely known.