Umar Farouk Abdulmutallab unsuccessfully attempted to detonate an explosive called pentaerythritol tetranitrate, or PETN, on Northwest Airlines Flight 253 on Christmas day. PETN was also used back in 2001 by Richard Reid, better known as the shoe bomber, who tried to destroy a plane over the Atlantic. That's two high-profile failures for PETN. So, why does it seem to be the explosive of choice for these terrorists? To help explain this is Jimmie Oxley, a professor of chemistry at the University of Rhode Island, whose specialty is explosives.