Sometimes you want your puppets to be dwarfed by their surroundings, but when you move the camera back, to get that long distance shot, you run out of set! You start seeing the walls of the room you are in! One solution is to suspend a sheet of glass between the camera and the set and PAINT onto the glass, to make it look like it's a continuation of the set!

It takes some skill to match the colours, and although this sort of "set extension" can be done digitally in post production, there's nothing quite like the immediacy of doing it in-camera like the pioneers!