The outer ear channels sound waves from the air around you inward through the ear canal. The ear canal terminates at a tightly stretched membrane, the eardrum, which is vibrated by the incoming sound waves.
Beyond the eardrum, in the middle ear chamber, are three tiny, linked bones called the middle ear or ossicles. The outer bone, the malleus (hammer), is attached to the eardrum. The inner bone, the stapes (stirrup), ends in a footplate which fits into the oval window, an opening in the wall of the bone which houses the inner ear. The center bone, the incus (anvil) connects the malleus and the stapes so that when one moves they all move.
The vibrations of the eardrum cause the bones in the middle ear to move back and forth like tiny levers. This lever action converts the large motions of the eardrum into the shorter, more forceful motions of the stapes. The movement of the stapes sets up motions in the fluid that fills the inner ear causing the hair cells immersed in the fluid to move. This movement stimulates the attached nerve cell to send a tiny impulse along the fibers of the auditory nerve to the brain where it is translated into the sensation we know as sound.