Looking for best response to (annoying, sexist, male) co-worker instructing me to "Smile!"

[ItsMeAndMyPumpkinNow]

I think I just frowned a bit and turned my face back to my workstation.

Am now fantasising about witty comebacks and googling for what the web has to offer. What are your faves, or retorts you have used?

Look him square on and go "Aha. Ah. Hmm." with gravitas, while nodding slowly. It doesn't mean anything but it will confuse him.