Season 6: Episode 138: The One With Unagi
Rachel and Phoebe are taking a self-defense class. Eager to help, Ross offers to teach them a concept he learned in karate called "Unagi", a state of total awareness. Determined to pass along his philosophy to the girls, Ross decides to surprise attack them to prove just how much they need his help. This set off a string of surprise attacks against each other. Ross's antics are brought to an end when he accidentally accosts two strangers on the street, having mistaken them for Rachel and Phoebe. Joey is broke and tries to earn money as a medical research volunteer. In attempt to get $2000 for an identical twin study, he persuades a fellow actor to pretend to be his twin. Monica and Chandler celebrate Valentine's Day a few weeks late and decide to give each other handmade presents. Monica feels guilty because Chandler gives her a mixed tape of romantic songs and she didn't make a gift. She attempts to make it up to him until she finds out that the mixed tape was an old birthday gift to Chandler from Janice.