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I'd say "depends." An untouched slice of pizza still in the box. maybe. Unopened cans of beans or boxes of pasta? Sure! Somebody's unfinished plate with their fork marks tracking through it? Nope nope nope!
I like the communitarian idea, and I like the anti-waste idea. I'm thinking Mental Floss is raising the idea of leftovers on peoples' pates just to get gross-out clicks.
I certainly agree with what Robin said. I consider myself pretty adventurous and probably over trusting of others but that's certainly something I wouldn't even consider doing with a stranger. With a friend, sure, but with somebody that you don't know at all?? Not a chance!