A friend of mine convinced a guy that she was a Russian foreign exchange student for SIX MONTHS. Starting near the beginning of the school year. He was an RA and would make rounds through our dorm. She changed her name, learned Russian phrases, invented a Russian family, and kept up the charade anytime he was around. 

On March 31, this guy was on duty to do rounds, so a small group of us gathered in a hallway so we would be sure to see him before the end of the night. We wound up seeing him just at midnight (April 1), and we mentioned that it was "'Natasha's' first April Fools Day." So we asked him to explain what April Fool's was to her. He said "Oh, well it's when you convince someone of something that isn't true." She said "so If I told you that I was American..." and he said "No! It has to be plausible, but I know you're Russian, so..." And she laughed and said "Do you not understand? I'm from Lansing."

Cue laughter, applause, etc.