Make a model to resemble the geysers found in nature. Take a piece of hardboard and drill a hole in the middle just wide enough to fit the neck of a large bottle of diet soda. Mold clay around the hole to resemble rocks and allow it to dry, but leave the hole exposed. Fit the hole over the open neck of a bottle of soda and balance it by placing the edges of the board on two stools.
Take a length of hose just wide enough to fit Mentos down it. Insert one end into the neck of the soda bottle. Bend the hose over and insert the entire tube of Mentos into the free end of the tube and hold them in place by placing your thumb over the end. Straighten the hose to allow the entire tube of Mentos to drop together. Do not fiddle around if one or two get stuck, as the reaction will happen very fast and is quite powerful. A spout of fizzing coke about 10 feet high will launch from the top of the bottle. It will start within a few seconds of dropping the Mentos and last about the same length of time, so quickly withdraw the tube as soon as the Mentos have dropped into the bottle and run away.
The reason this reaction takes place is because of the physical shape of the Mentos. Their surface is covered in tiny dimples which provide a surface on which the carbon dioxide can form into bubble. As the Mentos sink to the bottom of the bottle, the bubbles begin to form very rapidly at the bottom, releasing a large volume of gas beneath the main body of the liquid and forcing it out of the bottle.
Introduce kids to the scientific method by getting them to use a variety of different types of soda. Encourage them to predict which they think will cause the most violent reaction and which the least. Have them check if their hypothesis was correct. Get them to look at the list of ingredients on the back of each soda bottle and see if there is any relationship between the quantities of ingredients and the reaction rate. Given that the reaction is caused by the reaction between the carbon dioxide and the surface of the Mentos, you can investigate whether breaking the Mentos up into small pieces, and thus increasing their surface area, increases the power of the reaction. If it does not, you should ask students why this might be. You can also try leaving bottles of soda open for different lengths of time until much of the carbon dioxide has escaped and experimenting with how powerfully the different bottles will react. Less carbon dioxide should mean a less powerful reaction.