I'm 19 and I guess you could say that I don't know how to do it, and it's nothing to do with a lack of intelligence. When I was growing up I was told to do it but it made me so incredibly uncomfortable, with feeling like my head was going to explode and the dizziness it brought on, that I gave up trying. My guess is that's what Joey meant, not that he doesn't know how to, rather than it was so uncomfortable when he tried that it made him feel like he can't do it.