From Academic Kids
The stapes or stirrup is the stirrup-shaped small bone or ossicle in the middle ear which attaches the incus to the fenestra ovalis, the "oval window" which is adjacent to the vestibule of the inner ear. It is the smallest bone in the human body.
The stapes transmits the sound vibrations from the incus to the membrane of the fenestra ovalis.
The stapes is called the collumela in non-mammals, and is present in all species with ears.