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"It's established fact that blacks don't have the intellectual ability of whites."
"Fair enough."
?????!!
Tone is important. A small smile, a quick "hey, fair enough" can subtly tell the other person that you're done with the conversation. It can also be combined with a quick change in the subject without making it obvious that you wanted to change the subject.
Sorry, but comments like that don't deserve a smile, small or otherwise.
What they deserve is a punch in the face. Sadly, that's against the law, but it would certainly be "Fair enough."
It's not against the law to call someone out for being an ignorant ass.
If you can't afford to lose your job over it and you have kids to feed, I'll understand.
Ditto if your life would be in danger or you would be putting someone else's life in danger.
Otherwise, call them out on their crap.
People like that can't ever receive any encouragement in their beliefs. It's like showing a vicious animal a weakness. It only encourages them to do worse.