Take the top off of an empty plastic water bottle.
Place a funnel into the top of the water bottle.
Pour 8 oz. of water into a pot, and heat the water over medium heat until it is just hot.
Pour the hot water into the bottle through the funnel.
Pour 1/4 cup of sugar into the water through the funnel.
Pour 1 package of active dry yeast into the bottle through the funnel.
Place the opening of a balloon over the top of the bottle, sealing off the top.
Shake the bottle well, and allow it to sit on a counter or table top for 30 minutes. The yeast will react with the hot water and sugar to create gas, which will be visible when the balloon inflates.