Other countries have their own official 1.000000... kg cylinder, so they don't have to fly to Paris every time they want to measure something precisely. But since the Kilogram is the only standard, each country's knockoff has to be compared against it periodically. The United States has had its official kilogram, called K20 (the twentieth official copy), which resides in a government building in exurban Maryland, calibrated just once since 2000, and it's due for another calibration, says Zeina Jabbour, group leader for the NIST mass and force team. But calibration is a multimonth process, and security regulations since 2001 have made flying K20 to Paris a hassle. "We have to hand-carry the kilograms through the flight," Jabbour explains, "and it's hard to get through security and customs with a slug of metal, and tell people they cannot touch it." Even opening K20's customized suitcase in a "dusty airport" could compromise it, she says, "and if somebody insists on touching it, that's the end of the calibration."