This activity will lift spirits in your class, but there is little scientific explanation for what makes a hot air balloon rise. As you are teaching middle school scientists about density and convection, you can incorporate this activity. Another suggestion, this time more practical than educational, is to give groups permanent ink markers to decorate, instead of the suggested stickers or scraps of paper. This way, the center of gravity for the plastic bag is not affected. A terrific activity! It just needs to be supported with more solid background information.