If there's one thing that's more annoying than others in public, it's people telling you to smile when they have absolutely no business even getting in your way, let alone talking to you. Broad City, the show that recognizes that this is a problem for women everywhere has provided a solution so simple it's surprising no one's thought of it before.
You should just start practicing this as soon as you can, because it's a legit response to an awful directive. Why should anyone smile when walking down the street and why should your appearance matter to the strangers around you? You're out for some milk and a pack of cigarettes (or a $13 bottle of wine), not here to impress and inspire others. The show has already been picked up for a third season!
I would also suggest trying the method I use when this stuff happens to me. I was walking in downtown San Francisco one when a woman said "hey mister, you dropped something." I imagine she had no idea that I'm legally blind (which is why I am in constant squint/glare mode) also very neurotic about losing things, so she probably didn't expect me to drop to the ground and start frantically searching for whatever it is that I may have lost.
"What was it?" I asked her while crawling around the sidewalk on all fours. "Was it money?"
"It's your smile," she said back. "You've dropped your smile."
There was a pause and then I just shouted "Fuck off, lady." Needless to say, she dropped something, too.
Try it, you'll like it!
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