Since I'm not a professional and don't play one on TV, take this one with a grain of salt.
Some (many) of you may have seen sci-fi shows like "Sliders" that had plotlines where the main characters crossed over into other, parallel universes. I certainly enjoyed them because the possibilities were endless. But let's take a look at one of the most famous equations of all time:
What it means is that all of the energy "E" in our universe is exactly and permanently equal to all of the matter in our universe multiplied by the speed of light squared.
So, if you happen to be a supremely lucky individual who ventures to another universe, remember this: Don't bring back anything. Not a speck of dust, lunch (in your stomach), or breath in your lungs. If you do, all of Einstein-ian physics will no longer be valid and the Earth will probably explode... :-)