The main function of the middle ear is to convert vibrating air molecules in the outer ear into vibrating liquid in the inner ear. This is accomplished by bones that connect the eardrum to the inner ear. The air and the liquid of the inner ear have differing impedance, which is the resistance to vibration. The bones are called the malleus (hammer), incus (anvil), and stapes (stirrup), and they are connected in series via muscles and ligaments that allow relative movement. The purpose of the bones is to match the impedance so that the pressure waves are transmitted to the inner ear with as little power loss as possible. This avoids the tendency of a higher impedance material to reflect the sound away. An example of this is voices reflecting over the surface of a lake, rather than being transmitted into the water.