Carbonation
Carbonation is the process of dissolving carbon dioxide gas into a liquid to create bubbles.
Think of it this way
When you stir sugar into a glass of water, the sugar breaks apart and spreads through the liquid. Carbonation works the same way, but with gas instead of solid crystals. Tiny carbon dioxide bubbles push into the liquid and spread out through it. When the pressure drops, like when you open a can, the bubbles escape and rise to the top.
Explaining carbonation by grade level
When you open a soda bottle, you hear a hiss and see bubbles rise up. Those bubbles are a gas that was trapped in the drink. The gas stayed hidden in the liquid while the bottle was sealed tight. Once you open it, the gas escapes and makes that fizzy feeling on your tongue.