To make an Invisible Picture Glass.
This is a glass tumbler with an image on the bottom that is invisible until the glass is filled with water. Get a thick glass tumbler with a narrow bottom. Drop a transparent glass marble about 1 inch diameter into the glass and then put a ring of quick-drying adhesive liquid around the marble, where it touches the glass, to hold it firmly in place. Paste a small picture on the outside lower end of the glass tumbler so that the picture side faces toward the glass.
If you look down into the glass tumbler nothing can be seen because the picture is beyond the focal length of the marble. If the glass is filled with water, the picture appears suddenly and is magnified. This is because liquids have a higher refractive index than air which changes the focal length of the marble which acts as a convex lens to focus the picture into the eye so that it becomes visible.