Terms discussed: body weight
Poisons are not magical like in fairy tales, where just touching something or putting it in your mouth makes you drop down dead. A poison is a chemical that disrupts some crucial process or destroys a crucial organ in your body. If there is enough of the poison to disrupt the process or destroy the organ to a sufficient degree, you die. This is why children (and small animals) are so much more vulnerable to poisons than adults: a dose of amanitin that would destroy a child's entire liver, for example, would destroy the same number of liver cells in an adult; but this might be only a third of an adult-sized liver, so the adult would survive.
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The point is, you need to actually ingest some of a poisonous chemical for it to affect you.