In the animated movie Cars 2 the two agents shared this code phrase together so that they be sure the other one is an agent.
Holley: A Volkswagen Karmann Ghia has no radiator.
Finn: That's because it's air-cooled.
Holley: I'm Agent Shiftwell; Holley Shiftwell from the Tokyo Station. I have a message from London.
Why did two expert intelligence agents use a very obvious thing as their code phrase which every car can answer, just like Mater did later.
That's like I'm an agent and tell another one "Apple falls from the tree" and trust them as agents after they replied "That's because of the gravity effect."