"It's a PV. Personal Vaporizor. You would better know it by it's more common name: Electronic Cigarette. I built this one myself." I then slip off the cover of my box mod and show them the inside of it while I explain how it works and show the safety measures built into it so it won't blow up in my face.
When I get to the carto, I start into explaining the juice. "This is what gets vaporized." I show them a bottle of juice. "it's a mix of Vegetable Glycerin, Propylene Glycol, flavorings, water, and nicotine. It has approximately 4,000 fewer chemicals than a cigarette." In the event that they freak out about PG I tell them that it's a non-toxic base fluid that is used in everything from anti-freeze to asthma inhalers and everything in between, including hand lotions and flavor extracts for baking. Then I slide the cover back on it and vape!