Sugar and yeast inflate a balloon
because what is produced

As the yeast feeds on the sugar, it produces carbon dioxide. With no place to go but up, this gas slowly fills the balloon. A very similar process happens as bread rises. Carbon dioxide from yeast fills thousands of balloonlike bubbles in the dough.

Explanation:

i believe it is "they are not used up by reactions" as the answer to your question, text me back if this is correct because i just learned this.

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