Yeasts are a large family of microscopic fungi, referred to collectively by the scientific name Saccharomyces, which translates literally as "sugar mushroom." This is apt enough, given that yeasts and mushrooms are both fungi, and that yeasts feed on sugars. Their role in human food and beverage production derives from their digestion of the sugars in foods, and their production of carbon dioxide gas and alcohol as by-products. Depending on how the fermentation is controlled, the end result might be well-risen bread, a foaming glass of beer or a tangy cruet of vinegar.
Carbon dioxide production is the simplest aspect of yeast's usefulness to demonstrate for your class, and there are several ways to manipulate the process. First, fill a clean soda bottle partway with a warm sugar solution, and add some yeast. Shake the bottle, and stretch a balloon to cover the neck. Over a period of five to 10 minutes, the balloon will begin to inflate. The demonstration is most impressive with smaller balloons, so try this ahead of time and buy the smallest balloons that can be stretched over the soda bottles.
Vary the ingredients to introduce your kids to experimental methodology. Have them measure identical quantities of sugar solution into three different bottles, then add equal amounts of dry active yeast, fresh yeast and instant or bread-machine yeast. Have the kids record which works fastest, and which produces the most gas. You could also vary the amount of sugar in the solution, to test the effect on gas production. A third alternative is to change the food supply. Use sugar in one bottle, molasses in a second, artificial sweetener in a third, flour in a fourth. Discuss which worked best, and ask them why.
Another alternative is to vary the experiment's environmental conditions. Fill four bottles with sugar solution at different temperatures: frigid, cool, warm and boiling hot (pour the last one yourself). Add yeast, cap the bottles with balloons, and have the kids observe what happens. An alternative version of this experiment might include filling one bottle with cold solution and refrigerating it, a second with tepid solution and leaving it at room temperature, and a third with warm water and placing it on a sunny windowsill or a baseboard heater. Have the class observe all three, and speculate why temperature makes a difference.