three
o'clock
He set the other dial . Holding the cube in his hand he looked at his watch . `` It is six minutes before three o'clock . I shall now activate the mechanism -- by placing the cube on the platform -- at exactly three o'clock . Therefore , the cube should , at five minutes before three , vanish from my hand and appear on the platform , five minutes before I place it there . '' `` How can you place it there , then ? '' asked one of his colleagues .