Here is a video of one of our recent favorites. (Seriously, we had to immediately leave the house to purchase another liter of Diet Coke and go demonstrate this at Grandma's house!)
1 or 2 liter bottle of Diet Coke
Just drop the Mentos in and take a few steps back!
See! Even Carter and Natalie got to enjoy this one!
How does it work? Well, to explain it in terms a 7 year old would understand, it is a chemical reaction. The "fizz" in soda is made up of carbon dioxide. The same stuff you exhale. When the mint is dropped in the soda, it causes the carbon dioxide to rapidly separate itself from the rest of the drink and come rushing out the top!