hole

1. Origins, structure, and basic meaning

• Structure: 孔 means a hole; its on-reading is こう and its kun-reading is あな.
• Basic usage: 気孔(きこう) means a stoma in a leaf, while 瞳孔(どうこう) means the pupil of an eye.

2. Common pitfalls and similar expressions

• An opening in a leaf and the pupil of an eye
  • 葉の裏にある気孔 means stomata on the underside of a leaf. 猫の瞳孔 means a cat's pupils; the initial character changes the part being identified.
  • 孔 is read こう in both words. 気孔 and 瞳孔 begin with き and どう respectively.