Hard
A mouth that is old is going to be hard.
Go ahead and touch that old man's mouth. It's super hard feeling when you touch it.
こ
かた
None
You touch the old man's hard mouth and it crumbles away, revealing the face of a 子 (こ) inside.
"You pressed the hard skin holding me captive! Thank you!" the 子 says, as they break free from their hard mouth prison.