克莱因瓶
Klein’s bottle

The Klein bottle is a non-orientable surface; informally, it is a surface in which notions of left and right cannot be consistently defined.Simplifying things: A Möbius strip is a simpler example of a non-orientable object. That means it has no inside or outside. Add another aspect – having no boundaries – to it, it gets more complex and you end up with a Klein bottle.