Juul Labs has rapidly overtaken the U.S. e-cigarette market. In the past year, sales have skyrocketed nearly 800 percent and Juul is now valued at more than $15 billion. But the company faces lawsuits and FDA regulation, and its popularity among teens could threaten its future. CNBC gets a rare look inside Juul Labs and talks to the founders, James Monsees and Adam Bowen, to hear how the company is facing its challenges while trying to eliminate cigarettes from the face of the Earth.