A lesson I learned as a child. Thanks to a fictional television character named Eddie Haskell.
Asking my mother why this creepy kid always tells Mrs. Cleaver how lovely she looks.
And how Mrs. Cleaver wasn't so flattered.
Interesting. Eddie Haskell was such an archetype that he has a psychological phenomenon named after him: The Eddie Haskell effect. It's people who are overly polite to authority figures, but change into ruthless bullies when the authority figures are gone.
Psychology Today article on the Eddie Haskell effect