A rudder is a flat, movable piece attached to the back of a boat, ship, or airplane that controls which direction it goes. When you turn the rudder to the left, the vessel turns left. Turn it right, and the vessel goes right. On a boat, the rudder sits underwater at the stern (the rear), and the pilot controls it using a wheel or tiller. On an airplane, the rudder is the vertical flap on the tail fin.

The rudder works by pushing against water or air flowing past it. When you angle the rudder, it redirects that flow, which pushes the back of the vessel in the opposite direction, causing the whole craft to turn. It's like dragging your hand in the water while sitting in a canoe: your hand pushes the water one way, and the canoe turns the other way.

Rudders have been essential to navigation for thousands of years. Ancient ships used large steering oars as rudders. The invention of the stern-mounted rudder in medieval China revolutionized sailing, making ships easier to control and enabling longer voyages of exploration and trade.

People also use rudder metaphorically to mean something that provides direction or guidance, like saying a good teacher can be the rudder that steers a struggling student toward success.