Eyeball
The eye's root is the eyeball.
Go ahead and feel your eyeballs moving around in your head. Your eyes aren't just the part on the surface, they're the whole balls in there, attached at the root.
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You know who has some pretty good eyeballs? Gandhi (がん). Gandhi used his eyes to see the oppression around him and decided to do something about it. But it's all thanks to his eyeballs.
Look at Gandhi’s eyeball with your eyeball. It's pretty remarkable.