http://marshmallowchallenge.com
Great collaborative activity. Instead of the explicatory spin that Tom Wujec gives it, just ask participants the Jacotot questions: What did you see? What do you think of it? What can you make of it? There is a lot to see, so taking time with the first question (always a good idea) is especially important. I think it would pair well with Broken Squares, the Marshmallow game coming first.