I was remembering an episode from my past the other day and wanted to share it with you. I lived in Brazil for several years and taught ESL (English as a Second Language). The school where I taught specialized in British English. My first week, I was teaching a class of 4th and 5th grade students. One of the boys came up to me and said, "Teacher, do you have a rubber I can borrow?" (For any non-Americans who may not know, in US-English, "rubber" is slang for a condom.) Now, I knew this child was not asking me (in the middle of class, anyway)if he could borrow a condom. I didn't know what he wanted though, so I asked, "And, why do you need one?" He answered, gesturing to his paper on the desk, "I need to rub out an answer." Ah-ha..... in British English, rubber is slang for an eraser! I had one in my workbag and gave it to him.
When I taught my young adult class (more advanced) I took the opportunity to teach them both British and American slang. I could just see one of them coming to the US, needing to buy an eraser, going into a store and asking for a rubber.
It's fun discovering the differences in languages. I also found out (in a very embarrassing way) that in British English you do NOT call your behind your fanny. If you are interested in why, check out this