I really like for my kids to know that writing is not just about answers to questions or stories--that it can be fun too. Do people even read comics anymore? I feel like now that so many people read their news online, that might be something that is overlooked. I remember feeling like such a grown-up on Sundays sitting down on the couch with my parents, reading the newspaper (they automatically would pass me the comics! :)
I take a comic strip and simply put the balloon caption over the text that is already there and ask the kids to fill in what might be said. I love Marmaduke and the old Family Circle ones because it's just one square and usually it's pretty clear what is happening. When we get better at it, I move on to Garfield or Peanuts and they fill in more than one.
This is one of the Marmaduke ones-I can't even remember what the original joke was:
Go away, let go dog. Shoo, shoo!