Why is Superman vulnerable to Kryptonite?

It doesn't make any sense. How is it that his home planet is made up of Kryptonite yet he is vulnerable to it? Isn't it like saying most of the planet Earth is covered with water, yet water is very harmful to humans?Why is Superman vulnerable to Kryptonite?Isn''t it because he's so used to being free on our Earths gravity, that when he's faced with Kryptonite it emanates its' regular gravity or something like that. I dunno, that's what my thought has always been

sea doo

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