Reptile skin is covered in overlapping plates called scales. This layer of skin is good at keeping moisture inside, so reptiles can survive in hot, dry places. Skinks and snakes have smooth, flexible scales for burrowing or moving across the ground. The leathery scales of caimans are strengthened by bony plates on the back and belly while tortoises have a tough, warty covering on their head and legs. Many reptiles have rough, granule like scales that rise into spiked points along their back. These sharp spines are good for defense and some form beautiful crests which are useful in attracting a mate.