Click-drag to paint on the canvas. Shift-click-drag to cut a rectangular hole in the canvas, revealing a second canvas behind the first. You can paint through the hole, or cut a hole in the second canvas to reveal a third canvas, etc.

You can also move holes around with Ctrl-click-drag.

Apart from being a very contrived drawing tool, this applet plays with the concept of a "window". On a computer screen, we are used to thinking of windows as portals to separate and disjoint worlds. Here, however, the holes in each layer of the canvas are like windows in a wall. Two different windows show different parts of the same "world" beyond. Furthermore, to change what we see through each window, we must change the location of the window, rather than scroll its contents.