How can we explain his brother's autism to our four-year-old son?

July 14 |
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Photo by Mel Rowling, used with Creative Commons license.
We've got two boys. My stepson, A, is 11 and autistic (he's very intelligent, pretty high functioning, but his only language is a jumble of repetitive quotes from movies and random stuff he overhears) and my younger son, M, is four. M has just started to question why A isn't "like his friend's big brother." When M asked "Why won't A play with me? Does he not like me?" it struck us that we really didn't know how to explain autism in a way that a four-year-old would understand. I'd love some advice — anyone know any good kids books or resources that would help? — Gemma

Do you have any words of advice or know of a book or two that can help Gemma's situation?