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Behold, the squircle-oid!
This is the shape defined by the equation
\[ \left(\frac{x}{2}\right)^4 + y^4 + z^4 \leq 1 \]
I made it as part of a set of props for explaining Lᵖ norms. p=2 gives an ellipsoid, and in the limit p → ∞ you get a cuboid.