Just yesterday I saw a FB post claiming that aluminum foil causes Alzheimer's.
I replied with the Snopes link refuting the claim as "mostly false".
Got a response back about "not taking any chances", and "don't trust snopes", and "free country - so do what you want".
... then I replied with a warning about di-hydrogen-monoxide (with a link).
They agreed that that's dangerous stuff.
(forehead. desk. forehead. desk. forehead. desk...)