Crude jokes are impolite in public.
Bob is at a social function where he doesn't know many people.
Bob wants to break the ice and make friends.
Bob joins a group of people talking and tells a funny joke about dogs.
Bob joins a group of people talking and tells a joke based on a racial stereotype.
The other people laugh and Bob makes new friends.
The other people in the group are offended and avoid Bob the rest of the night.