I am a sarcastic person. I mean a really sarcastic person. I can't help it; I just am! When I was pregnant, my husband and I always joked that our unborn daughter better have a good sense of humor if she was going to survive with us as parents. This "sense of humor" included having to put up with sarcasm.
One trait of people who have autism or Asperger's is that they can have a really hard time distinguishing when somebody is being sarcastic -- they can be very literal. For example, if you ask an Aspie if the "cat's got your tongue," they may wonder if you somehow think a cat ran off with their tongue.
To help my daughter out in this area -- and because I just cannot turn off my sarcasm -- I continue to use my sarcasm non-stop around her. Because of this, she has gotten pretty good about figuring out when I'm being sarcastic. She doesn't always succeed, however. There was the one time that I was commenting that she was the most beautiful girl in the world, and she laughed and said, "You're being sarcastic, right?" I had to tell her I wasn't -- that she really was the most beautiful (I would have thought she would have known that one!).
As she's getting older, however, she's picking up on the sarcasm. One time when I was driving, a driver almost plowed into our car. Always (okay, usually) careful not to curse in the car with my daughter in it, I commented, "Nice driving!" My daughter piped up from the back seat, "That must be sarcasm because that driving was horrible!"
Now that my daughter has turned eight, she's not only getting good at understanding sarcasm, she's actually beginning to use it herself. Oftentimes when I tell her something, she'll roll her eyes and say, "I know" (while making the word "know" into a two-syllable word). She's also been know to retort something along of the lines of, "For the five thousandth time, I said...." This may not sound like a big deal to parents of kids who are not on the spectrum, but it's a very big deal for a child on the spectrum.
Nevertheless, I sometimes wonder why I took so much time in teaching sarcasm. It might not have been one of my most thought-out plans!