This week we build some self-intersecting Boy surfaces:
http://teach.hilderbuild.com/tiki-index.php?page=Paper%20Boy%27s
http://www.southernct.edu/~fields/topology/Boys_surface.html
Polyhedral versions:
http://plus.maths.org/content/os/issue27/features/mathart/index
http://www.savoir-sans-frontieres.com/JPP/telechargeables/English/Topo_the_world_eng.pdf page 48 (the Topologicon is available in many languages besides English)
http://www.southernct.edu/~fields/topology/Boys_surface.html