True story. Post Office on High Holborn in London earlier today. Tourist wants to send an airmail letter but is struggling with the blue "par avion" sticker. A member of staff shows him how to peel it off its plastic backing.
The guy then starts trying to peel the stamp he's been given, but is struggling with that too and can't seem to make it work. The member of staff informs him that he needs to lick it and then stick it on the envelope.