I'm at a supermarket and a young girl is fussy because her parents didn't buy her something she wanted. She's not having a tantrum, she's just sulking and glaring.

The mother and cashier tell her "Pretty girls don't get angry."
The girl frowns as she seems to struggle with the concept.

They're effectively telling her "You're angry, so you're not pretty." combined with society's baseline value of "girls HAVE to be pretty, or they're nothing" that is spoon-fed to women from day 1. They're setting her up for a lifetime of smiling when men in the street tell her to. To compromise her own feelings for the benefit of perfect strangers.

I was too stunned to admonish the adult women passing their internalized misogynism on to the next generation, and I'm angry with myself for not speaking up.