A circle is a shape with constant [b]radius[/b]. This shape has a constant [b]diameter[/b]. This idea can be used to drill (almost) square holes and make coins which work in slot machines. [url]http://www.cut-the-knot.org/do_you_know/cwidth.shtml[/url] [url]http://web.mat.bham.ac.uk/C.J.Sangwin/howroundcom/roundness/applications.html[/url]