Why do mentos fizz up and pop right out of a coke bottle

What's happening is the Mento is causing the carbonation in the bottle to release nearly all at once, causing the pressure in the bottle to increase drastically. This pressure causes the Coke to shoot out of the top, or if there's no hole the pressure makes the top shoot off or something to that effect.

Personally I like to drill holes in the top and use a little bit of wire to drop the Mento in. Then it looks really spectacular : D

From online - "After a lot of debate, scientists are now saying that the primary cause of Coke & Mentos geysers is a physical reaction, not a chemical reaction. Their explanation is this process called nucleation. All the carbon dioxide in the soda - all that fizz - is squeezed into the liquid and looking for a way out."