A knife sharpener is a tool or device used to maintain the sharpness of a knife's blade. It works by grinding the blade's edge against a hard surface, usually made of stone, ceramic, or metal, to restore its cutting ability. There are various types of knife sharpeners, including manual sharpeners, electric sharpeners, and sharpening stones, each designed to suit different needs and skill levels. Regular sharpening helps keep knives efficient and safe to use.