As he approached the security checkpoint at Washington Dulles International Airport one afternoon last April, Jon Pratt felt on edge. Stuffed in his camera bag were four solid-metal cylinders, the sorts of objects guaranteed to draw the scrutiny of wary Homeland Security TSA staff. Each cylinder weighed exactly one kilogram. One of them—a gleaming platinum-iridium alloy about half the size of a can of tuna—was worth at least $40,000. (The price of platinum currently hovers around $1,000 per troy ounce, a common unit for precious metals.) The other three consisted of finely machined stainless steel.