25) 牙

ya

tooth

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

25ax) 穴

xue 

hole, cave

25a) 穿

chuan

penetrate; wear

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

25bx) 口

kou

25b) 呀

ya

ah, oh

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