This episode is one of the underrated bunch of seasons 9 and 10. Excellent start to the last season of Friends.
Monica, Chandler and Phoebe discover that they can eavesdrop on Ross & Charlie and Joey & Rachel through the walls of the hotel which leads to the hilarious Rachel/Joey/Ross imagination scene and Chandler's hair jokes towards Monica are priceless. Ross tells Joey on the plane ride back to New York about him and Charlie, but Joey can't quite yet. Later, Ross walks in on Joey and Rachel kissing leaving us with a cliffhanger...Meanwhile Phoebe funnily has to break up with Mike's girlfriend, Precious after their reconciliation.