Here's why:

* Neutralizes Stomach Acid: Calcium carbonate reacts with stomach acid (hydrochloric acid) to form calcium chloride, water, and carbon dioxide gas. This reaction neutralizes the excess acid, relieving heartburn and indigestion.

* Common Antacid Ingredient: Calcium carbonate is a very common ingredient in antacids, often found in brands like Tums and Rolaids.

* Other Ingredients: While calcium carbonate is the most common, some antacids may use other ingredients like magnesium hydroxide (Mg(OH)2) or aluminum hydroxide (Al(OH)3) for their antacid properties.

Important Note: If you're experiencing frequent or severe heartburn, it's important to consult a doctor to rule out any underlying medical conditions.