They do that? Seriously? That sucks.
My parents say they want to learn, but they never really took the time to learn. They did learn very little years ago when they volunteered to host parties for my deaf group from church. They were never committed to it.
Today, my mom decided to start learning again because of something that happened in our family... but she's a little slow. She does wish that she learned sign language a long time ago when I was a kid.
You know what you should do? When you have a deaf friend in the house, start signing and don't use your voice. Do it in front of your parents. Tell each other jokes and then laugh about it so that it makes your parents wonder what you're saying. If they ask, tell them to learn sign language. 
