For what it seems a very long time ago, I found this Scientific Experiment which I used for my 3rd and 4th grades back in 2011. Results were OK but I took a high risk with little kids.
This November 2012 I retook it for my ever lovely 9th graders (3rd grade of Secundary). Results were better that expected since I was expecting a bonfire. I have some troublemakers.
First we arranged the pieces and wrote the instructions.
Then in teams, I gave them the material and we did. Of course, some wax ended on the floor, some rubber bands snapped, some cardboard arrow were burned around the edges… minor details.
On the sweet side, they love it and I was relief that I didn’t have to use a bucket of water.
More info in one of my first posts: Experiments for Science-related topics, part 1