Tie and tape a piece of yarn or string to the center of a straw, dowel rod, or cardboard cleaner's hanger tube. Attach two identical balloons with clothes pins, clips, or pins to the ends of the rod. Be sure to make them balance. Take off one of the balloons and inflate it fully. Then tie it off and reattach it to the rod.
Air has mass.
The air makes the balloon bring the balance arm down. Inflate two identical balloons to about the same size. Tie a string about 30 cm long to each. Hold them about 3 cm from each other and blow through the space between them. Blowing causes the air pressure between the balloons to decrease creating a need for the space to be filled and the pressure of the air on the outside of the balloons pushed them together.