Flexi Says: Fish are covered with scales. Scales are overlapping tissues, like shingles on a roof. They reduce friction with the water. They also provide a flexible covering that lets fish move their body to swim. Fish have gills. Gills are organs behind the head that absorb oxygen from water. Water enters through the mouth, passes over the gills, and then exits the body. Fish typically have a stream-lined body. This reduces water resistance. Most fish have fins. Fins function like paddles or rudders. They help fish swim and navigate in the water. Most fish have a swim bladder. This is a balloon-like organ containing gas. By inflating or deflating their swim bladder, fish can rise or sink in the water.