Yeast is placed into a Ziplock bag with warm water and sugar and sealed so no oxygen will enter the bag overnight the bag swells up from what gas give it off by the yeast

The growing yeast gives off carbon dioxide gas.

Explanation:

Ancient Egypt first use this idea of growing yeast.

Growing yeast like to eat sugar, starch.

When they eat these food materials, they release a gas called carbon dioxide gas which gradually forms lots of tiny bubbles.

Using this idea, bread is made. As it form tinny bubbles in the bread dough, it pop during the time of baking and leaving tiny holes.

So for their growth and development, water and sugar is an important factor.