Part 1: Write down 4-6 opening lines. Don't overthink it; just write what comes to your mind. After you have done this, pick one of the lines that you think you could write about, and write it on a small scrap of paper. I will then collect everyone's scraps, put them together in a bag, and have each person choose a new line for themselves.
Part 2: Beginning with this new line, write a story, or a poem, or whatever comes to your mind in response to this line. Again, don't think too much about it, just try to let it flow. Write for 15 minutes.