Toggling between two different images creates a spatial impression. This is the 3D effect Björn Barnekow once showed me. Here accidentally rediscovered. Look at the styrofoam cup. See how it shakes much less as it is closer to the projection-surface and such the paralax shift is much smaller? Just by jumping back and forth between these two images our perception system creates the 3D effect. We see what we want to see, or as Heinz von Foerster put it: "It is not seeing is believing but believing is seing."