Friends Season 6 Episode 17 : The One With Unagi (a.k.a. The One With The Mix Tape)
Rachel and Phoebe take self-defense classes. Ross, who had previously practiced karate, tries to attack them by surprise and convince them that they lack unagi, supposedly a state of "total awareness". (In reality, as pointed out by Phoebe and Rachel, unagi is just a Japanese dish.) Chandler and Monica celebrate Valentine's Day several days after the actual date because of their schedules, but they forget to make each other home-made gifts: Monica gives Chandler one of Phoebe's "sock bunnies", and Chandler gives Monica a mixtape, which was originally a present from Janice. Joey decides to get money through medical research but learns that the only study available is for identical twins which pays $2000. As a result, Joey tries to find someone to play his twin.