Because water offers more resistance than air, it can be energy-intensive to move through it. Bony fishes overcome resistance by moving their bodies, tail, and fins in wavelike motions, generating force against the water. Swimming technique varies, depending on a fish's lifestyle. Long bony fishes, such as eels, move their whole body in wavelike motions that allow them to move forward or backward, maneuvering into small spaces. Bony fishes that need more speed, such as trout, move just the back half of their bodies, making them faster but less maneuverable. Long-distance swimmers, such as tuna, maintain a rigid body with the tail (caudal fin) and tail connector (peduncle) doing all the motion. Some bony fishes use their paired side fins (pectorals), such as puffers who swivel them back and forth. Many bony fishes have a sac-like structure (gas bladder) that allows them to keep the right buoyancy by inflating or deflating it with gases.