When I was a kid, my English teach would write a couple sentences on the overhead. It would be something like this:
"Mary walked into her house only to see that it had been completely destroyed. The dog was no where to be found and though no one replied when she called out, "hello?", she had a feeling she wasn't alone..."
The goal of the exercise was to continue the story only using one or two sentences at a time. The first person branches their next two sentences off of mine to further the story. The next person bases theirs off of the first person and so on until the story keeps rolling and people can take it any way they want.
So go ahead. Be creative. I set the story up for murder, mayhem, aliens, vampires, bullies, fire, etc. Add in your own flair and see where the story goes.