Liquids

• Natural carbonation in liquids is a result of fermentation and a byproduct of yeast growth. Yeast consumes sugar and, in the process of digesting it, gives off carbon dioxide. This gas is invisible while the liquid is held under pressure, but becomes visible when the pressure is relieved. Open a bottle of champagne, pour a glass and watch the bubbles rise. Pop the top on a shaken can of soda and it will spray foam everywhere. Pull the tap on a beer keg and a creamy head of foam will form in the glass. The bubbles and the foam are visible signs of carbonation. Mechanical carbonation by the injection of carbon dioxide into an unfermented liquid yields the same results.
  
  

Solids

• Solids can be carbonated, too. Look at a slice of bread. All those little holes that give the bread its texture are visible signs of carbon dioxide bubbles in the dough, and there's no way to tell if this occurred from fermentation or injection. The dough solidifies around the bubbles and the gas escapes during the baking process, leaving the holes behind. Even candy can be carbonated. If sodium bicarbonate is mixed with citric acid in the dry ingredients, there are no visible signs and the powder can be formed into any desired shape. Put it in the mouth, though, and as soon as saliva hits it, it turns to foam and becomes the candy once sold as Pop Rocks.
  
  

Human Body

• Carbonation isn't a normal process in the human body, but it can produce a visible effect there. Drinking too much carbonated liquid too quickly can cause audible signs, such as burping and stomach rumbling (borborygmi), and invisible signs, such as a stomach ache, but it can also cause bloating, visible as abdominal distention.
  
  

Concrete

• Carbonation occurs in concrete when carbon dioxide in the air combines with water to form carbonic acid. This weak acid dissolves some of the chemical bonds in the concrete and weakens it. The visible sign of this is sand. Unless it's a seawall at the beach, growing piles of sand around concrete means it's coming apart and needs to be repaired.
  
  

Rocks

• Rocks also suffer from carbonation, and the visible signs of this are called weathering or exfoliation, the flaking off of particles. This is how soil is formed, so the carbonation of rocks is a beneficial result of a destructive process.