Pupil
Inside your eye stands a tiny village that makes up your pupil. That's the black part of your eye!
Oh, did you think your pupil was a hole? No, no, you misunderstand. That's where the village stands. It's full of little people who whisper to your brain what you should see.
どう, とう
ひとみ
None
All of the people in your pupil eat doughnuts (どう) all day long. That's why your eye is shaped like a doughnut, with the "hole" (village) as their home.
Even if you're a pupil non-believer, at least you can see how it's shaped like a doughnut hole, right?