I got a set of Johnson Cups and Balls and brass wand for my Birthday last week (my wife loves me!). Last night I polished them up using Flitz, probably the best metal polish ever made. I placed one of the red balls on top of the cup and from a downward view of this setup, it appeared as though the cup had a glass botton and there was a LARGE red ball inside the cup. This is due to the convex shape of the bottom of the cup. It even had a 3D effect to it. I don't see a way to use this illusion in a performance (yet) but I thought it was an interesting illusion. Is this a known feature of this apparatus?