Howard said he'd held an engagement party for Jimmy Kimmel and his fiancée Molly over the break and invited all of his (but not necessarily Jimmy's) friends.
As the guests gathered around the cake, Gary watched Robin work herself into a duet between Billy Joel and Jimmy Fallon. "Two other people started singing and you joined in," Gary said to Robin.
But Robin said she only joined the pair at Howard's behest. "You urged me to join in!" she said.
Howard laughed that Robin didn't need much encouragement. "You went for an hour and a half," he said.
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."