Shake the bottle to dissolve the sugar. Open the bottle and sprinkle some of the yeast onto the surface of the water. Put the opening of the balloon over the mouth of the bottle. Pull the stem part of the balloon down so that it will not come off easily. As the yeast starts to work, it will release carbon dioxide.
Yeast grows quickly in water that feels a little warm.
Wrap the bottle with a towel to help keep the heat in. Every now and then, unwrap the bottle and look for bubbles of carbon dioxide coming up from the dividing yeast cells. As the yeast works, it will break down the sugar. The rest is turned into alcohol. The yeast cells will grow and bud, producing daughter cells.