Fish swim by flexing their bodies and tail back and forth. Fish stretch or expand their muscles on one side of their body, while relaxing the muscles on the other side. This motion moves them forward through the water. Fish use their back fin, called the caudal fin, to help push them through the water. The fish's other fins help it steer. Fish have 5 types of fins. They are the pectoral, pelvic, dorsal, anal and caudal fins. Animals and engineered vehicles are both subjected to the same physical forces that govern their performance when moving through water. Animals have a vast variety of propelling mechanisms connected with swimming modes, body morphologies and performance levels as a result of evolution. Propulsive modes have been characterised by biologists based on the anatomy (axial, appendicular), kinematics (anguilliform, carangiform, thunniform) and propulsive forces (drag-based, lift-based, acceleration reaction) of the animals. The various swimming modes are linked to different performance indices (such as speed, acceleration and manoeuvrability) that are dependent on the animal's ecology.