The claw folds up into a club when the animal is about to throw a punch. The whole appendage itself is very thick, up to five times as thick as the adjacent appendages, according to the researchers. The impact zone, the outer part of the claw, contains the mineral hydroxyapatite, a thin material that is also found in vertebrates' bones and teeth. It's only 50 to 70 nanometers thick. Normally, such a thin layer would fracture easily — but it's backed by a supportive matrix of chitosan, which disperses the impact and prevents any crack from spreading. And the third region, which runs along the sides of the claw, is even less stiff, helping disperse the blow even further.