Observations and results
When the yeast get warm water and some food to eat (in the form of sugar), they will become active. And as they eat the sugar and break it down for food, they release carbon dioxide, which fills up the balloon.
Yeast is actually a type of fungus related to mushrooms. The type of yeast used for baking is usually Saccharomyces cerevisiae, but it is one of more than 1,000 species of yeast. Our own bodies actually have plenty of yeast species that live peacefully alongside (and inside) us!
Other foods, such as cheese, also make use of tiny creatures and their little life cycles. Instead of yeast, cheese is brought to you in part by bacteria—but these are carefully controlled and healthful types of bacteria, so no need to worry about eating it.
Share your living yeast observations and results! Leave a comment below or share your photos and feedback on Scientific American's Facebook page.
Rinse out the bottle and recycle or reuse it. Reuse the balloon.
More to explore
"Yeast Does DNA Tricks to Live in Us" from Scientific American
"Mixed Cultures: Art, Science and Cheese" from Scientific American
"Bread Science 101" overview from the Exploratorium
"Microorganisms" overview from the Children's University of Manchester
I'm a Scientist: Kitchen by DK Publishing, ages 4–8
The Science Chef: 100 fun food experiments and recipes for kids by Joan D'Amico, ages 9–12
Sink or Swim: Muscle Versus Fat
What you'll need
• Cooked piece of meat that has both lean meat and fat on it (such as a pork chop or steak)
• Knife to cut the meat
• Piece of bread
• Large clear glass cup or bowl