For Monkey Boy's fourth grade science project he chose to experiment with carbon dioxide... the classic dropping Mentos mints into carbonated cola drinks - resulting in a sticky, messy explosion of foaming soda. He did a great job doing the research and learning all the "scientific" stuff behind the experiment, and was excited to get to work on the actual fun parts of the project - the EXPLOSION. He learned that companies add slightly more carbon to the diet sodas to make up for the bitter taste - making the diet sodas have a higher reach in the explosion. So, we bought several brands of diet and regular sodas and set to making it happen. The winner, and not coincidentally the worst tasting soda was: Walmart's generic brand of Diet Cola. The loser of the explosion-height race was regular Royal Crown Cola. Some of the sweeter tasting versions like Pepsi and Coca-Cola Classic were the better tasting, but not very high in their mint-reaction measurements.
We did this same experiment a couple of years ago with all the neighborhood kids in attendance. Military Man dropped in several mints per soda bottle, resulting in enormous explosions... the kids all thought it was so cool! We scaled it down quite a bit since we needed to have a more accurate measurement in the height of the explosions, so we only used two mints per bottle.