Gas Experiment: yeast, sugar, and water

Considering I have a PHD in engineering, I don't do enough experiments with the children.

I'm trying to change that.

I did a really fun experiment recently.  Very simply, I poured yeast, sugar, and water into a glass bottle with a narrow neck. I then placed a large balloon over the neck.  The yeast, sugar, and water began to react and the yeast started generating carbon dioxide.

And voila...over the course of 24 hours, we observed the following:

I confess I was surprised how long the yeast reaction continued, and how much the balloon blew up!

Lots of fun.