My son is taking American Sign Language as his foreign language in high school (where the hell was this when I was in high school?), and he's become pretty good at it. I don't know how conversant he is, but he's learned a lot in a short time. He came home from his second or third day of school able to spell his name and some other basics. My fiancee and I have been trying to learn some of it, little by little, partly because we didn't have this opportunity growing up but mostly because we don't want him using it against us.
Anyway, some of the signs are very similar; we all got a good laugh when, in a moment of educational frustration, one of us blurted out, "That's not 'Grandma,' that's 'vomit'!" (Note: the signs for "Grandma" and "vomit" are quite similar; grandmas and vomit themselves are not.)