An oldie but still a goodie.
Fill a glass with a clear, carbonated beverage. Drop 4 or 5 raisins in the glass. Watch the dancing raisins. Why do they dance?
Since the surface of the raisins is rough, tiny bubbles of carbon dioxide gas are attracted to it, causing it to be carried upward by the more dense fluid surrounding it. Once the raisins reach the top, the bubbles pop upon exposure to the air. This makes the raisins more dense, causing them to sink.
This same principle can be applied to the Mentos and Coke reaction.