Sexism is something people joke around about a lot, not knowing the affect it can have on someone.
Women belong in the kitchen!
I really can't believe that some people have the audacity to say something like that to a person just minding their own business!
There was also this one time someone stared me down and talked about me under their breath at a skate park, just because I was a girl!
CAN YOU BELIEVE IT?!
You can't skate!!
You're a girl!!!!
Someone might ask, "Why don't you just tell them that what they are doing isn't right?"
To answer your question, we try to tell them non-stop, but after we confront them, they don't take what they did wrong into account. Instead of fixing their mistakes they still perceive themselves as correct and tell us to "calm down," while then proceeding to be sexist again.