This Klein bottle will blow your mind! According to Wikipedia, the Klein bottle is an example of a shape with a "non-orientable" surface... That is to say, its insides are its outsides and vice versa! You can think of it as the three dimensional equivalent of the two-dimensional Möbius strip.

I designed this simple Klein bottle to be easily printable at various scales, and included models for the two halves of the bottle so you can show off the insides.