Speculation has been growing that North Korea may be ready to carry out its sixth nuclear test as soon as this weekend. If so - it would coincide with the anniversary of the birth of the nation's founder, Kim Il-sung. The BBC's John Sudworth is in the North Korean capital Pyongyang - he's among a group of journalists who have been invited into the secretive country to witness the anniversary celebrations. He's been speaking to passengers on the Pyongyang metro system.