Streamlined objects disturb the flow of the fluid less and have a lower pressure drag. If the animal is not to be swept backward it must exert a thrust to balance the drag. If the animal wants to move forward, its thrust must be greater than the drag. When a fish undulates its body and tail, a thrust is created. The same applies to birds, bats, insects and airplanes as they fly through the air.
Humans create thrust when they swim by dragging their arms and legs through the water.
Thrust can also create lift. The thrust that propels a tuna, whale, or porpoise rapidly through the water is provided by the lift created as its tail oscillates back and forth. Viscosity Fluids such as air and water are different from solids in their ability to flow. How rapidly a fluid flows, or is deformed by a given driving force, is determined by the fluid s viscosity. Water is much more viscous than air.