Many times as a parent I am asked, how is it like to have a child with Autism? (In my case three children) How do they act? Timothy Archibald is a photographer and he also has a son with Autism. He decided to compile a few pictures of the things his son does on a daily basis. You can take a look to see some of the things autistic children do, some of these pictures are powerful and touching. Source 2
They look 'sad' all the time because from their eyes people do not understand them. I am not only talking verbally but behaviorally too. To say that they are frustrated is putting it mildly. When they take their bowl of cereal and empty it on the floor and then start eating it off the floor, what is our initial reaction....well we yell at them saying it is wrong (Sometimes we don't even say it, we just yell at them) but they do not see it as wrong and guess what you never explained to them it is wrong. When I say explain, I don't mean that they dirtied the floor because they don't see that as a problem either but why they shouldn't eat off the floor for example. Saying mummy or daddy does not like you throwing things on the floor means nothing to them for they normally don't have empathy and definitely not sympathy, so using that angle like you would with a child without Autism is not going to work.
I rambled too long but I think you understand what I mean. I could say more but I don't feel up to it now. Let's just say that I have not even scratched the surface yet.