It is a lonely existence to be a child with a disability which no-one can see or understand, you exasperate your teachers, you disappoint your parents, and worst of all you know that you are not just stupid.
I guess my feeling is is that if you're going to make a joke, that's fine, but you should also sort of stand behind it, you know? A joke should be more than a joke, it should be a point that you're trying to make.