Afternoon conversation: In which I am put in my place

me: [at grocery store] "I think we should get some grapes."

Sam: "I love grapes."

me: "I know."

Sam: "How do you know?"

me: "Mommies know everything."

Sam: "That's not true."

me: "Of course it is."

Sam: "Well, maybe you know everything about your kids, but you don't know *everything.*"

me: "Oh?"

Sam: "I mean, look at you. You only teach college. It's not like you teach grad school or anything."