牙

ya

tooth

a molar is a tooth that grows in the corners of the mouth

穴

xue

hole, cave

roof/dwelling (mian) + something splitting open (ba)

穿

chuan

penetrate, pass through

wear (clothes)

cave (xue) + tooth (ya)

a molar penetrates gums creating a kind of a cave; one also wears clothes by "penetrating" through sleeves and trouser legs

口

kou

呀

ya

ah, oh

that's the sound one makes when one's mouth is open wide enough to see the molars;

to convey emotions, soften tone, or make sentences more conversational