There was an old Yiddish joke.
A secret agent being briefed. "You will go to Budapest. [tedious details of how he gets there elided) You will travel to the apartment house at 21 Hevizi Ut; there you will meet with Agent Cohen. You will tell him 'The sky is blue!', and he will reply 'The grass is green!' Having thus identified himself, you will pass these papers on to him and take some from him in return, which you will deliver to our embassy in the usual manner. Any questions?" Spy travels to Budapest, finds the apartment house, and looks at the directory. What do you know, there are two apartments labeled Cohen. He trudges up the stairs to the second floor and knocks on the door.
"Who is it?"
"Is that Mr Cohen?"
"Yah, who vants to know?"
"The sky is blue!"
"Oh, you vant Cohen der shpy. Fourt' floor."
(address picked at random from a google map of budapest because I couldn't remember the joke's original address, which was probably made up in any case)