A propeller is a device with rotating blades that pushes air or water backward to move a vehicle forward. Think of how a ceiling fan moves air around a room: a propeller works the same way, but instead of cooling you down, it creates thrust that drives an airplane through the sky or a boat through water.

Propellers have two or more blades arranged like the arms of a pinwheel, all attached to a central hub. When the propeller spins rapidly, each blade cuts through the air or water at an angle, pushing it backward. Newton's third law of motion explains what happens next: for every action, there's an equal and opposite reaction. As the propeller shoves air or water one direction, the vehicle gets pushed in the opposite direction.

Airplane propellers might spin 2,000 times per minute or faster. Boat propellers work underwater, where they need fewer rotations because water is much denser than air. Submarines, cargo ships, and speedboats all use propellers to move through water.

Before propellers, ships relied entirely on sails or oars, and human flight wasn't possible. The invention of efficient propellers in the late 1800s changed transportation forever, making both aviation and modern shipping possible.