It's been posted here a dozen times or so, but is always worthy of a repost

"dangerous"?? I'd call it cool, interesting, even neat-o. But hardly dangerous.
Advice: before showing your friends, figure out which ones are most gullible. "Did you see the report on foxnews? You know how when you get metal in your mouth, it has a certain taste to it? Well, they figured out that if you put metals in the soil near grape vines, it's absorbed into the grapes, giving them more of a sweet taste, plus it keeps insects from eating the grapes. Of course, they can't get away with it in this country, but MSNBC and CNN discovered that in poor 3rd world countries, where our grapes are imported from this time of year, they put heavy metals like mercury in the soil as a sort of fertilizer. Here - I'll show you how much metal is in a grape. You know how you're not supposed to put metal in a microwave? Watch this."
I've suckered dozens of people on this; several vowing to never eat grapes again.

(Of course, I tell them it's a joke afterward)