Deep inside your ear
Behind your eardrum is a tiny space called the middle ear. Three tiny bones live there: the hammer, the anvil and the stirrup.
When sound hits your eardrum, these bones pass the shaking along, deeper into your ear.
The stirrup, called the stapes (STAY-peez), is the smallest bone in your body. It looks like the stirrup a horse rider puts a foot in. The stapedius is tied to it and named after it.
Tiny, tinier, tiniest
The stapedius is the smallest skeletal muscle in your body. Its muscle part is only about 1 millimeter long, much smaller than a grain of rice.
Scientists measure it in different ways, and some count its little tendon too. But any way you measure, it's tiny!
Try it: find a ruler and look for the smallest marks. The space between two of those tiny marks is about how long this muscle is.
A volume knob for loud sounds
When a sound is very loud, your brain sends a quick message, and the stapedius tightens. It holds the stirrup steady, so less of the loud shaking gets through.
It works all on its own, and in both ears at once, even if the loud sound is only on one side.
It also tightens a little when you talk, so your own voice doesn't sound too loud to you.
It needs your help too
This little muscle can't block every loud sound. A sudden bang, like a firework, happens faster than it can tighten.
Very loud sounds, especially for a long time, can hurt your hearing. So cover your ears or wear earplugs or earmuffs in very loud places, and keep headphones turned down.