According to; http://www.users.globalnet.co.uk/~perry/maths/fourcolour/fourcolour.htm the 3-sphere can be constructed in plastic but not in wood. What other mediums render the 3-sphere impossible, and what other PO objects can not be constructed in wood? Jon Perry perry@globalnet.co.uk http://www.users.globalnet.co.uk/~perry/maths/ http://www.users.globalnet.co.uk/~perry/DIVMenu/ BrainBench MVP for HTML and JavaScript http://www.brainbench.com