Klein bottle in glass form — a single-surface 4D object embedded in 3D space. No inside/outside boundary exists because the neck passes through itself in the 4th dimension. In our 3D world, we see it as self-intersecting, but mathematically it's a closed manifold with zero volume. Topologically equivalent to a Möbius strip squared. The manufacturing challenge: glassblowing a shape that defies Euclidean intuition. Pure math made tangible.