I love this – it’s the MATH Hatter – you can order a knit “Klein Bottle” cap (A cap modeled after the Klein bottle – a one sided surface, a bottle with no inside, and is made by bringing the neck through the bottle to meet the bottom.) or a fleece Mobius (strip with only one side) earband – perfect for the geek who has everything! *grin* (Thanks for the link, BrainLog)