Why does a bottled carbonated beverage fizz when you take the cap off?

The carbonated beverage contains carbon dioxide gas dissolved in it and much of the gas was made soluble into the beverage thanks to pressure. As soon as you open the bottle, there is a decrease in pressure in the bottle and the solubility of the gas immediately decreases (less gas is soluble in the beverage) and hence, you see the bubbles coming out. :)