Start with a surface filled with random dots, cut out a small square, move it a bit to the left, fill the resulting white space on the right with new dots.
The image pair is viewed with a stereoscope so that the left eye sees the original image, and the right eye sees the transformed image: both images fuse to one single image where a square seems to float above the dotted surface._(You may even arrive in seeing the floating square without a stereoscope, if you look with each eye at one image, and squint so that both images fuse to one; but this exercise is very straining for the eyes!)
The reason to use random dots was to prove that the brain can produce a 3-dimensional image, even when the original image doesn’t contain any suggestive forms.