Propulsion is the force that pushes or drives something forward. When a rocket blasts into space, propulsion from its burning fuel pushes it upward against gravity. When you ride a bicycle, your legs provide the propulsion that moves you down the street.

Different vehicles use different types of propulsion: cars use engine power, sailboats use wind, and jet planes use hot gases shooting backward to thrust themselves forward. Even swimming involves propulsion: when you push water backward with your arms and legs, the water pushes you forward in return.

Scientists and engineers spend careers studying propulsion systems to make vehicles faster, more efficient, or able to go places nothing has gone before. A propulsion system is whatever mechanism creates that forward-moving force, whether it's a propeller, a jet engine, or the chemical reactions in a rocket.

When you throw a ball, your arm provides the propulsion. When a squid shoots through the ocean, it uses water propulsion by squirting water out behind itself. Understanding propulsion means understanding how things move from one place to another, and how the right amount of force in the right direction can take you where you want to go.