I helped with my sons K class a few years ago. The teacher had the kids decorate styrofoam cups and put them in the hall "to dry". While they were busy with class work I snuck through the hall and took all of the cups to the kitchen. I put them upside down on cookie sheets and put them in the oven for 2-3 minutes at 200 and they shrunk to about 1/2 the size (you have to be careful or the bottom my totally disappear). I then filled them with green m&ms and put them back in the hall where I found them. My son talked about what they found for weeks. It was truly precious and he still doesn't know how it happened.