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If you know anything about baking bread, you know that in order to make it rise, you need yeast．It looks like other powdery ingredients, but it is still alive．When it is dry, it is dormant, but when it is moist and warm, it comes to life．
Yeast is similar to plants, but it can't make its own food the way plants do．Instead, it feeds on sugar．As yeast breaks down the sugar to make energy, a chemical reaction called fermentation takes place．This creates alcohol and carbon dioxide as waste．The carbon dioxide appears as little bubbles of gas．These bubbles are what cause bread dough to rise and baked bread to have its light, spongy texture.
Experiment
Materials：two packets of yeast, two plastic bottles, two balloons, warm water, granulated sugar, a tablespoon, a funnel.
• Pour a cup of very warm water into each bottle．Place a funnel over the mouth of bottle l and add two tablespoons of sugar．Place the cap on the bottle and shake it until the sugar dissolves.
• Open the bottle and put the funnel over the mouth again．Add the yeast and replace the cap．Swirl the mixture around in the bottle until the yeast dissolves．The water should be cloudy．Follow the same procedure to add yeast—but not sugar—to bottle 2, the control bottle.
• Open each bottle and slide the end of a balloon over the bottle's mouth．Make sure that the balloons create a tight seal．
• Put the bottles in a warm place, like a sunny windowsill．After about 20 minutes, you will notice that balloon l has inflated．It trapped the carbon dioxide that the yeast produced during fermentation．Balloon 2 will not have inflated, because yeast does not ferment and produce carbon dioxide without sugars to feed on.
Word List
dormant： not active
swirl： move around in a circle
inflated： filled with air
You Practice
I．Answer the following questions.
1）How can we make yeast come to life?
2）What is the main difference between yeast and plants?
3）In the experiment, why does balloon 2 not inflate?
4）What is fermentation?
5）Do you like this experiment? Why or why not?
II．Word power—Use these words from the reading in sentences of your own.
1）yeast ____________________
2）inflate ____________________
3）dormant ____________________
4）cloudy ____________________
5）rise ____________________
III．True / False / Not Given—Write T if the sentence is true, F if it is not true, or NG if there is no information about this sentence.
____ 1）Yeast feeds on sugar．
____ 2）Most bread contains yeast．
____ 3）It is possible to make bread without yeast．
____ 4）Yeast is a kind of plant．
____ 5）In the experiment, both bottles contain the same ingredients．