Fallacy of Composition

Assuming that something true of part of a whole must also be true of the whole.

Example

Dental fillings contain mercury and mercury is poisonous. Therefore dental fillings are poisonous.

This is not correct because dental amalgam and methylmercury are completely different chemical compounds that both include mercury atoms but have different properties. Just like how water contains contains oxygen atoms, but water is nothing like the oxygen that we breathe.

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