What is the name of the smallest bone in the human body?

The stapes - about three millimetres, shaped like a stirrup, and the last of the three tiny bones carrying sound from the eardrum to the inner ear.

They exist to solve an impedance problem. Sound arriving in air has to be transferred into fluid, and almost all of it would simply reflect off the surface; the chain concentrates the force collected by the large eardrum onto the much smaller oval window, multiplying the pressure enough to get the energy in. Without it hearing drops by around 30 decibels, which is the difference between conversation and muffled noise.

Two of those bones used to be a jaw. In reptiles the jaw joint is made of bones that, across the transition to mammals, migrated into the ear to become the malleus and incus - traceable through a sequence of fossils and repeated in the human embryo. Two tiny muscles also contract to stiffen the chain against loud sound, which is why your own voice sounds quieter to you than to anyone else, and why the reflex is too slow to protect against a gunshot.