To demonstrate how yeast works, all you need is two cups of warm water, two to three tablespoons of honey, one package of fast rising yeast, a 22-ounce plastic soda bottle, and one well-stretched medium balloon. A funnel may be helpful as well.
This demonstration will last more than one day. Have the students write a hypothesis about what they expect to happen, and explain that they should take daily notes. A detailed, constructed response should be written at the end of the demonstration. The amount of detail required should be within the expectations of the students' grade level.
To start, completely dissolve the honey into the water. I like to use honey as it dissolves faster and does not precipitate out as sugar sometimes does. Pour the water and honey mixture into the bottle. Use a funnel if it will make the pouring easier. Next, add the yeast to the bottle. Place the balloon over the top of the bottle and keep it in a warm place but out of direct sunlight.
Observe the bottle several times a day over the next few days. You should observe the yeast starting to foam up within ten minutes. As the yeast starts to foam, the balloon should begin to inflate. Observe the reactions of the yeast over the next few days.
After several days, remove the balloon and let the students smell the mixture. There should be a definite foul smell. The odor is the result of the digestion process. As yeast eat and digest the honey mixture, they excrete. The gas they produce makes wonderful light and airy bread. However, when left in a soda bottle, you have a malodorous gas that smells somewhat of methane and sulfur. If there is a microscope available, you may want to have the students view the yeast under the microscope before and after the demonstration.