“[A]lthough the sphere... is curved intrinsically because you can't draw a precise map of the atlas of the world on a piece of paper without distortion, if you have something on a scroll.. you... unroll that and there's no distortion. A cylinder looks curved, but it's... flat from the point of view of Gaussian curvature. It curves in one direction, but not in the other, and it's important that... both change.”