The ends of an egg have higher curvature than the sides. (The sides are not "flat" however.) When pressure is applied to the egg by the striking object (like the edge of a spoon, frying pan or a knuckle), bending stress is induced in the shell. When an end is struck, the stress is reduced because of the shape of the shell, which provides a compressive opposing force. Compare the stress on the curved roof of a domed house with that of a flat roof covering the same area. The curved roof is stronger because the slope of the roof allows some of the downward force to be opposed by compressive stress rather than bending stress, which tends to be higher.
