OK, maybe this isn't exactly a workshop topic but some of you have great answers in this forum! I have a picture of a small trick, a cube about 2" square. The cube has a short, capped rod piercing it vertically and this rod is held, thumb on the bottom end, and index finger on the top, probably to allow the cube to spin. Each of the four faces of the cube (those not pierced by the rod) have a 1.5" circle and the picture shows two of these, one of which is dark-colored and another that is red. Do you know the name of this trick or its effect?

Can you show us the pic?

Sorry I couldn't upload the picture (couldn't find any instructions in the tech forum and have never sent a picture before) but meanwhile George Robinson (Collector's Workshop) identified the trick as one made by Carl Bremma. It is a forcing block operated by a weight inside so any one of four colors can be forced.
Dave