Two things:1. Humans can do this, too. It's called daydreaming. There is nothing unique or interesting or, least of all, funny about it.
2. What the hell is Marmaduke doing in a store?
Maybe Tolstoy started this way.
Golly, and now a joke about how health care is expensive.
This is actually another good example of using Twitter to tell a joke about something else, which in this case is the uncoolness of Sally and her friends.
Now Sassy deserves to die, too.
Lots of comics have been making jokes about Twitter lately, and they've mostly been lame. What sets this xkcd apart is that it's not so much about Twitter as it is about swine flu. Which is to say, most of the comics have been using Twitter as a joke in and of itself, whereas this cartoon is using Twitter as a vehicle for a joke about something else. It works much better.