An impossible figure that locally (but only locally!) looks like a torus.
Impossible Torus
See also
Ambihelical Hexnut, Impossible Figure, TorusThis entry contributed by Margherita Barile
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Cite this as:
Barile, Margherita. "Impossible Torus." From MathWorld--A Wolfram Web Resource, created by Eric W. Weisstein. https://mathworld.wolfram.com/ImpossibleTorus.html