A convex surface (facing us) has positive curvature and a concave surface has a negative curvature. A flat surface has zero curvature. For example, a banana peel has positive curvature outside and negative curvature inside. Take a paper and roll it as cylinder. Length wise, you can draw a straight line. Hence, lengthwise its curvature is zero. But, across you can only draw circle or curve. In this direction it has positive curvature. If you multiply '+' and '0', you get 0. Hence the curvature of cylinder at any point is 'zero'. This is called "Gaussian curvature" of cylinder. Because of 'zero' curvature, you can roll a flat paper into cylinder and back. Next take a sphere, all sides, it is curved convexly. Hence at any point, it has positive Gaussian curvature. Hence spherical surface cannot be converted into flat one. Take an orange. Peel its skin. Put on the floor. stomp on it. It will not be
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