Anvil

three tiny bones are missing. put them back.

an ear cut open on an anatomy plate: put its three bones back and a click goes from silent to loud, then play a fork, a horn and an anvil through the chain you built

Air is easy to move and the fluid of the inner ear is not, so most sound would bounce off it. The three bones fix that: the drum is about 17 times the area of the stapes footplate, and the hammer and anvil are a 1.3 to 1 lever, so the fluid gets about 22 times the pressure. Loud, long sounds make the stapedius muscle brace the chain, but it takes about 30 milliseconds to start, so a sudden bang is through before it moves.

Concepts: hearing, anatomy.

An interactive you play with one thumb, on Wundr.