Continuity mistake: This is a mistake within a mistake: Niles and Frazier are trying to get the wrapped presents out of F.'s apartment and replace them with others, to play a trick on their dad. They are getting completely mixed up, and presents are being taken out and then returned by mistake, mostly by Frasier. At one point Niles has a pile of gifts which he puts down on the table behind the couch - the shot shows him holding them, but the next shot shows a completely different pile of presents, even though it is supposed to be the same pile. Frasier picks this pile up. It soon changes again in the next shots.
Answer: Nanny G (Nanette Guzman - played by Laurie Metcaf) has been a children's entertainer for twenty years, and tells Frasier, "Do you know what it's like to play the same character for 20 years?!" The joke is that Kelsey Grammer has been playing Frasier Crane since 1984, when the character first appeared on "Cheers", so Kelsey in real life has been doing just that!
Super Grover ★