John was drinking a bottle of Diet Coke, and he had a Mentos mint in his mouth. He accidentally back-washed and the mint went into the bottle. He screamed, dropped the bottle, and stepped back about ten feet.
Why did he do this?
Answer
When he loses the Mentos into the soda, the gelatin and gum arabic from the dissolving candy breaks the surface tension. This disrupts the water mesh so that it may take less work to expand and form new bubbles.
The reaction is physical, not chemical.
Basically, the ingredients in the Mentos and soda are similar to the reaction of baking soda and vinegar, and cause an eruption.