
 If It Looks Like a Sphere...: Science News Online, June 14, 2003 (Site not responding. Last check: 20071011) 
  In contrast, a coffee cup is not a sphere but instead a distorted version of a doughnut, and a pretzel can be considered a doughnut with three holes instead of one. 
  Even though spheres and tori sit in threedimensional space, mathematicians focus on their surfaces and so view them as twodimensional, unlike solid balls and filledin doughnuts, which are threedimensional. 
  For instance, just as the sphere is the twodimensional boundary of the threedimensional ball, mathematicians have defined the hypersphere as the threedimensional boundary of the fourdimensional ball—a space that's hard to visualize but that can nevertheless be analyzed mathematically. 
 www.sciencenews.org /20030614/bob10.asp (2652 words) 
