Phoebe lends Joey the happiest dog in the world to cheer him up after his affection for Rachel - but manages to bum him out after only one day with him.
She also lends Rachel a birthing video for Rachel to watch before she gives birth.
So, it's Valentines day and Chandler see's the video labelled Candy and Cookie on the table - Porn.
Infact, not porn, just a rather vivid video of a woman giving birth. Which destroys the valentines evening, for Monica, Rachel and Chandler.
Mona dumps Ross on Valentines day for neglecting to tell her about Rachel moving in with him.