In this episode we discover something easy to do, but not so easy to understand or explain :) We could say we work with two displays. One is the basic Processing display. We also have another display. We can copy any of the two displays into the other. We use this as a way to take a photograph of the main display. Things are changing in the main display. When something interesting happens (we release the mouse button), we copy whatever we see in the main display to the other display. This other display is just an image stored in a variable.
At any moment we can draw that copied display into the main display. We do this many times per second.
This, which may sound slightly confusing, results in a very simple effect: we are now able to move an object over the screen, and make it stick on the background whenever we click.