A prison contains twenty inmates and a warden. The warden has built a room containing two pointless levers, each of which can be either in the up position or the down position, but no other state. The warden tells the inmates that he will select a random inmate to visit the room, and allow them to change the position of one or both of the levers - and indeed, when they do so, they must change the state of at least one lever. He will continue to select inmates in this manner after random time intervals indefinitely. At any time, an inmate may approach the warden and tell him that all inmates have visited the room at least once. If the inmate is correct, they all go free; if he is wrong, they are all summarily executed. He allows the inmates to strategise together before he begins, but he doesn't tell them the original position of the levers or which inmate he will select first. The inmates devise a plan to escape as soon as possible without risking their lives. What is their plan?