The mentos has tiny indents all over its surface. These are called nucleation sites. Carbon dioxide from the soda build up in these spots, forming bubbles. The mentos sinks and the the bubbles rise. What is in the way of the bubbles? Liquid. The bubbles rising to the top force the liquid out the bottle in an awesome eruption! Now would this be a chemical reaction? Why or why not?
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-Any soda will work but diet doesn't leave a sticky mess behind.
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*Optional geyser tube is recommended but not needed. The tube allows for a high spray of the soda in a narrow flow. Also you have less of a chance to get drenched with the soda! If you use the tube, please follow directions on the packaging.