The eggs of birds resist external loading via composite structure.
"The eggshells of birds are mechanically impressive devices, surprisingly resistant to external loading; Vincent (1990), though, complained about how little we understand them, muttering at 'half-boiled notions' in the literature. They're mostly mineral but have a critical 2-4 percent of organic matter, making them into composites. Still, cracks can propagate, of which fact the chick takes advantage to get out--before pushing, it pecks around a circle so it can then break the egg along the dotted line." (Vogel 2003:340)
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