I've seen this phenomenon described in a nice way, although I haven't done the construction myself. Build a frame, say a cube with 12 rods. Place an object inside the frame, and tie it with strings to the frame, for example 8 strings to the vertices of the cube. The strings need to be slack enough to provide the wiggle room required. Rotate the object 720 degrees. It is then possible to manipulate the strings while holding the frame and object fixed, so as to untwist the strings and restore the original configuration. On the other hand, if the object is rotated 360 degrees, this manipulation cannot be performed. Gene __________________________________ Do you Yahoo!? Yahoo! Small Business $15K Web Design Giveaway http://promotions.yahoo.com/design_giveaway/