4x01 "The One with the Jellyfish"

During a trip to the beach, Monica, accompanied by Joey and Chandler, is stung by a jellyfish; the only remedy they can think of is for someone to pee on the sting. Chandler complies. Ross and Rachel get back together until Ross cannot stand Rachel thinking he has taken full responsibility for the flaws in their relationship. Phoebe gets to know her birth mother.