At approximately 5:38, Eric Cartman says to nanny Stella "If you love children so much, how come you're not a mommy?" However, whether or not the nanny had children was never mentioned, so Eric could not have known that she wasn't a mother. The reason Cartman knows this, and many other details about the nanny's life, is because it serves as the punchline for a joke where he offends the nanny so much to the point where she storms out of the house and quits.