One student group hypothesized that the dry baker's yeast must be alive. the temperature change, odor, and bubble formation, they said, indicated which characteristic of life.

The changing temperature, odor and bubble formation all indicate that the yeast is responding to external stimuli. This is a characteristic of living organisms. They will respond to changes in their environment. The changes that occur here are a result of the yeast using the sugars in the dough to carry out respiration.