• Hold down the SHIFT key and move across the squares to make them black.
• Click on a square to reverse its color.
• Click while holding SHIFT to paint (fill in) the figure.

The painting algorithm works like this: To "paint" starting at row R, column C,

1. If that point is already black, quit.
2. Move LEFT until hitting a wall or boundary.
3. Paint(R-1, C) — the point above.
4. Paint(R, C+1) — the point to the right.
5. Paint(R+1, C) — the point below.