In biology, a scale is a small rigid plate that grows out of an animal's skin to provide protection. Scales are quite common and have evolved multiple times with varying structure and function.
Fish scales are bony and covered with a smooth transparent tegument to improve the flow of water over them. Reptile scales are more like fingernail. Birds also have scales, commonly on their feet, and their feathers are thought to have been derived from modified scales. A few mammals also have scales, such as the pangolin, and these are originally derived from hair.
Sharks do not have scales, instead being covered with small denticles which are similar in structure to teeth. Some other fish are also scaleless or have incomplete scale coverage.