In this simple activity, learners see the production of a gas, which visibly fills up a balloon placed over the neck of a bottle. Inside the bottle, water and an effervescent antacid tablet are combined. The chemistry of the reaction is that an acid and base in the antacid tablet react once the tablet is dissolved in water, producing carbon dioxide gas. While gases are visible in liquids as bubbles, many gases aren't visible in air. Seeing the balloon inflate helps convince learners that a chemical reaction can produce products we cannot see. Extensions include experimenting with the amount of water, and the temperature of the water.