I remember when I grew up we used to call each other mongo as a joke and not meaning anything bad about it, but then on one occasion a teacher overheard us and told us that she was sad to hear people use the word that way because she had a grandchild with Down’s syndrome. I think this was one of the first times I learned that words said to one person can be hurtful to someone else even if they were not the person you said it to. And after that I’ve tried to be more conscious about which words I use in public.