Klein bottle

This is a 3-dimensional representation of the Klein bottle, a non-orientable surface without a boundary. Like the Möbius strip this object only has one side, so you can go from one point to its opposite without crossing a boundary.

UPDATE: File "Klein_bottle_without_barriers_Thingiverse.stl" does not have a wall through the tube, so that you can fill in a e. g. a fluid.