Crabs are also known as decapods because they have ten legs.  Their first pair of legs is modified into claws.  Crabs cannot easily grow larger like most animals because of their hard shells or exoskeletons.  Long before molting, a crab will reabsorb some of the calcium carbonate from its shell.  A new softer shell develops below the old one, and this may take several weeks.  A day before the molting process happens a crab will absorb ocean water, making it swell or puff up.  This swelling causes the old exoskeleton to crack along a seam around its body.  The actual process of molting takes about fifteen minutes.