How does a shoe function?

That's an interesting way to put it. a shoe does not "function." all you do is put the shoe on your foot and tie the shoe laces.

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A shoe functions to be a protective covering of an object. It does not always have to be a foot that it protects. It can be the end of a pole, a horse's foot, or even a rigid plate tightened against the inside of a brake drum to produce a braking action.