That one is easy: Say thank you with a Mona Lisa-smile, then go on about your day.

It's a pure formalism, because any interpretation of what they said, how they said it, of your own thoughts or feelings on it, is 100% pure speculation. Thus you simply accept it formally with a mild smile and a "thank you" and leave everything else (thinking about it, discussing it, etc.) for a later time.

It's very similar to how you're supposed to take/accept criticism.