This is a very clever and miniature version of the Houdini Water Torture effect.
You show a small block with a picture of Houdini on it and placed it in a tube. Water is then poured into the tube, such the Houdini is totally submerged under water, and the tube is capped. The image of Houdini is clearly visible in the tube. You then cover the tube with a borrowed piece of paper and when the paper is removed, Houdini magically disappears!