A mug shaped like a Klein bottle, thus having zero volume and one-sided surface. What a great piece of tableware to drink your morning coffee from! The only problem is that a four dimensional space is needed to create one because the surface has to pass through itself without a hole to create the shape. Also, having a surface without a hole can make it quite hard to pour coffee inside the thing. If you are ready to accept a more palpable three dimensional approximation of the Klein bottle, you can check out http://www.kleinbottle.com/ which sells these most intriguing pieces of spatial geometry.