The Lemonade and Mentos Fountain
Lemonade or any carbonated drink is a mixture of chemicals, one of which is dissolved carbon dioxide gas. When sparkling lemonade is made, the carbon dioxide is forced into solution under pressure and the bottle is sealed. The CO2 stays dissolved as long as the pressure is kept constant - this is an example of Henry's Law which states:
the pressure of the gas determines the concentration of a gas dissolved in a liquid.
When the bottle is opened the pressure is lowered and the CO2 becomes less soluble, and comes out of solution - which is why if you look at the side of the bottle when you unscrew the cap, you see thousands of bubble appear apparently from nowhere...