A stir bar is a small, cylindrical magnet used in laboratories to mix liquids. It is typically made of a magnetic material and is often coated with a non-reactive substance like Teflon. When placed in a liquid and subjected to a rotating magnetic field from a stir plate, the stir bar spins, creating a vortex that helps to evenly mix the solution. Stir bars come in various sizes and shapes, depending on the volume and viscosity of the liquid being mixed. They are commonly used in chemistry and biology experiments to ensure thorough mixing without the need for manual stirring, which can introduce contaminants.