Hah, I always only eat one, and I just carefully open the package and then fold the top over it again and put it back (I keep the box on its side, so they're flat--they last longer that way). I never worry about it, it's basically dry food, and they last forever unopened. They're not vacuum sealed or anything, so open ones don't seem to go bad much quicker...but I'm kind of just guessing--never gotten sick though, and I've never seen them grow mold or anything weird. If you're really paranoid you could just fold it over and tape it or something.
 Well, not *longer*, but they "bend" if you leave them standing for a long time.
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