[1][2] At the last minute, Monica decides to throw a Halloween party; enthusiasm diminishes markedly when she announces that everyone needs to come in costume.
Rachel's maternal instinct kicks in when the first trick-or-treaters arrive, and she spends the evening handing out candy... most of it to a smart little girl who figures out how to charm her.
Ursula's fiancé Eric (Sean Penn), a 2nd-grade schoolteacher, arrives first and immediately slaps Phoebe's butt; after working through his embarrassment, they begin talking.
It becomes clear after some conversation that Ursula has been lying to him, basically returning his own answers to him about her age, pastimes, history and employment, including herself being a teacher at the fictional "Top-Secret Elementary School for the Children of Spies".
This, compounded by Eric's sudden urge to be impulsive and romantic, resulted in the two planning to be married barely three weeks after meeting.