"Simple enough," he answered.  "We Allied aviators have a secret system of signals, something like Freemasonry.  When we come near another plane that seems to be one of our own, we make a certain dip of our plane.  That's like asking for the countersign.  If the other fellow's all right he makes a certain signal in return.  If he doesn't do it the first time, we try again, because there's always a chance that he hasn't noticed our signal, or is too busy in handling his plane to give the reply.  But if after two or three times we don't get the countersign, we know the fellow's a Hun and we open up on him."