Cut the base from your plastic bottle.
Cut the neck off of one of your balloons. Stretch the balloon to fit over the open bottom of your plastic bottle. Tape the balloon to the sides of the bottle.
Insert the plastic straw into the other balloon. Wrap the rubber band around the neck of the balloon, securing the straw in place. Roll your clay into a small ball, approximately two inches in diameter. Make a hole in the center and push the straw through the hole. Insert the straw so that it extends at least three inches above the clay ball. Press the clay around the straw to seal out gaps and air.
Place the balloon end of the straw into the top of the water bottle. Use the clay to seal the opening. To watch your model in action, pull down and release the balloon at the base of the bottle.