An exoskeleton is an external skeleton on an animal, contrasting with an endoskeleton that is internal. The exoskeleton serves to support and protect the animal’s body. The exoskeleton can have a number of other functions including excretion, sensing, and feeding. Exoskeletons contain chitin as a construction material. The exoskeleton attaches to muscles from apodemes. These can contain elastic energy for jumping and are six times as strong as tendons. When an organism outgrows its exoskeleton it must molt, or shed, its old exoskeleton and grow a new one. At first the new skeleton is soft, so the animal pumps itself up to let the skeleton harden at maximum size.