Just take two Möbius strips; Every has only one loop to be a boundary. Straighten out People loops into circles, and Permit the strips distort into cross-caps. Gluing the circles with each other will create a completely new, closed manifold without having boundary, the Klein bottle. Closing the surface area http://deanvkswz.tinyblogging.com/An-Unbiased-View-of-manifold-valve-28373094