Despite the importance of beta-sheets as building blocks in proteins and also toxic elements in the pathological disorders, ranging from Alzheimers disease to mad cow disease, the principles underlying their stability are not well understood. Non-random beta-sheet propensities of amino acids have been revealed both by their distinct statistical preferences within known protein structures and by the relative thermodynamic scales through the experimental host-guest systems. However, recent fitting analysis has proved that a native beta-sheet conforms to a minimal surface with zero mean curvature, like the physical model of soap films.