The arapaima's scaly coat is basically the fish version of a bulletproof vest, which helps protect them from cold-blooded killers like piranhas. The scales comprise two layers: a hard exterior made of minerals, and a flexible interior made of collagen. Unlike human-made bulletproof vests, however, those aren't fused together with a third adhesive layer—the two layers of each arapaima scale actually grow together. This makes it ideal for pressure absorption and range of motion, which researchers hope can help inform advances in materials science.