A paddle wheel is a large wheel with flat blades (called paddles) attached around its edge that pushes a boat through water. When the wheel turns, the paddles dip into the water and push against it, moving the boat forward, much like how you push water backward with your hands when swimming to move yourself forward.

Before modern propellers were invented, paddle wheels powered the first steamboats in the early 1800s. These boats had enormous wheels mounted on their sides or at the back (the stern), sometimes two or three stories tall. As steam engines turned the wheels, the paddles churned through the water, pushing massive riverboats up and down the Mississippi River and even across oceans. The showboats and riverboats you see in old movies about the American frontier typically had giant paddle wheels, often painted white, splashing dramatically as they moved.

Paddle wheels weren't very efficient compared to propellers: they wasted energy, broke easily in rough seas, and were vulnerable in naval battles. But they worked well in shallow rivers and calm waters, making them perfect for exploring America's inland waterways. Today, you'll still see paddle wheel boats offering tours on rivers, keeping alive a piece of transportation history. Some people also build small paddle wheel boats as science projects to demonstrate how pushing against water creates forward motion.