What happens when you sprinkle dry yeast on a banana slice and seal it in a bag? Yeast is a fungus that feeds on organic matter. Over a few days it breaks down the fruit in a visible way. You place two banana slices in separate sealed bags. One gets a half teaspoon of yeast on top. The other stays plain as a control. Each day for three to four days you observe and record what happens to both slices. The yeast-covered banana breaks down noticeably faster. This same process explains why bread dough rises. Yeast reacts with sugars and releases carbon dioxide gas.