It all started when, while visiting Monday's Tonight Show, Fey put in a special request.
"I know you're interviewing me here, and you're doing a pretty good job, but I would also like to interview me," said the former 30 Rock star.
"I think I know what you're talking about and I have a way to make that happen. It's time for a lip flip," said Fallon, referring to a popular bit of his.
Sure enough, Fallon's lips are then superimposed over Fey's, and vice versa.
Fallon, using Fey's lips, proceeded to ask Fey, "I heard you like to sing, is that true?"
To which Fey, utilizing Fallon's lips, replied, "Well, you know me. I love to sing, sing, sing, sing, sing, sing, sing, sing!"
All the silly talk eventually led to a duet of "Endless Love," and naturally, hilarity ensued.
Want proof? Well, then look no further than the clip above.
Meanwhile, Fey's new film, Muppets Most Wanted, hits theaters on March 21.
(E! and The Tonight Show Starring Jimmy Fallon are both part of the NBCUniversal family.)