During the first world war, there was a prisoner who had been captured. When brought to the army headquarters, the prisoner was told that he had a choice to remain silent or say something. He was also told that if he did say something, he would be shot if he spoke the truth or hanged if he had lied. The war prisoner did in fact make a single statement; he said only a phrase and they had to let him go unscathed. What did the prisoner say to get out free?