The purpose of this activity is to allow students to think critically to answer an EXTREMELY fun question. In this case, does Diet Coke really provide the best mentos/soda eruption? The students will need to use the scientific method to determine a way to answer that question. They will also need to design a test to empirically show that Diet Coke is the most explosive soda.
This is a great activity to use as a starting point to introduce concepts in chemistry, the scientific method, research skills, and general science. I've used this experiment several times and the students can never get enough of it.