“Magic Balloon,” Friend, Mar. 1975, 44
Magic Balloon
Fill an empty soda water bottle three-fourths full of warm water. Then with a funnel or paper cone add three to five tablespoons of sugar to the water. Now add a small package of yeast to solution and fit a balloon over the mouth of the bottle. Because the yeast is releasing carbon dioxide the balloon will start to inflate. Leave it for a couple of days in a dark, warm place and see how big the balloon becomes.