Blow up a balloon with yeast:
- Tell the kids that you have some yeast. Yeast is a fungus, like tiny one-celled mushrooms. They eat sugar, and "burp" air (CO2). The kids will laugh.
- Pour a packet of yeast into an empty water bottle. Ask the kids what plants and animals need in order to grow. They'll probably come up with food and water.
- Water the yeast: pour in 1/2 cup water.
- Next feed them: add 1/4 cup sugar.
- Swirl the bottle around to mix.
- Now catch the CO2, to prove that the yeast really makes some. Stretch a balloon over the top of the bottle. I used yellow smiley-face balloons, and the kids call it the "yeast buddy".
- The kids were very excited, expecting the balloon to fill immediately. The balloon started to stand upright in about 15 min, and continued to grow larger over the next few hours.
- For older kids, you could experiment with different amounts of yeast and sugar. Measure the balloon every 15 minutes at first, recording how quickly it grows.