Key concepts
Land: push against a surface
Muscles pull on bones or body segments. Feet, claws, scales or the whole body push backward against the ground, producing a forward reaction force.
Air: generate lift and thrust
Wings redirect air downward and backward. Wing shape, flapping rate and air resistance determine whether an animal climbs, glides or slows.
Water: streamline and push fluid
Tails, fins and pulsing bells accelerate water backward. Streamlined bodies reduce drag; buoyancy reduces the effort needed to stay at a chosen depth.
Cells: reshape or beat tiny structures
Amoebas extend pseudopods using flowing cytoplasm. Paramecia coordinate many cilia in waves to push surrounding water.