Howard said he'd held an engagement party for Jimmy Kimmel and his fiancee, Molly, over the break, and invited all his -- but not necessarily Jimmy's -- friends.
As the party gathered around the cake, Gary watched Robin horn in on a duet: "Two other people started singing and you joined in."
Robin said she only joined the pair -- Billy Joel and Jimmy Fallon -- at Howard's behest: "You urged me to join in!"
Howard laughed that Robin didn't need much encouragement: "You went for an hour and a half."), so Robin conceded: "You know what I thought after that? 'I should join a glee club!'"
Gary laughed: "You know what I thought after that? 'If Robin would stop singing, I could hear Billy Joel.'"
Robin insisted that even Billy had commended her harmonies, leading Howard to deadpan: "He told me I was handsome."