Thanks, Clark and Thane! Nice links. Didn't know about all those sphericons. It would be fun to play with solid models of them. Now that I see them, I'm sure the shape I was trying to think of was in fact the (4,1) sphericon: Rotate a square about a diagonal, cut it in half through that diagonal, rotate one half 90 degrees with respect to the other and glue them back together again. That nice and simple sphericon would be especially fun to play with. There are probably plenty of games waiting to be invented that could be based on this shape. --Dan ________________________________________________________________________________________ "Outside of a dog, a book is man's best friend. Inside of a dog, it's too dark to read." --Groucho Marx