5. Now recharge one of the balloons (across your head) and put it against one side of the door, and gently let go. The balloon should "stick" to the wall.

How does it work?
It’s all because of static electricity.  Static electricity is a force that is created when objects lose or gain electrons.  When you rub the balloon across the top of your head, it picks up electrons from your hair and becomes negatively charged.  The balloon is now attracted to positively charged objects so that it can “get rid” of the extra electrons.  Conversely, the balloon repels other negatively charged objects (like the other balloon) because it does not want any more electrons.

Natural Occurring and Practical Applications of Static Electricity
Lightning is probably the most dramatic natural form of static electricity.  When the water droplets in a cloud swirl around in the air, they can gain electrons making the cloud negatively charged.  The cloud then tries to send its extra electrons to a positively charged object; this could be another cloud or the ground.  The zap you sometimes get when you walk across a carpet and touch a doorknob, or the crackle you sometimes hear when you comb your hair are also examples of static electricity.

One example of using static electricity is in painting cars. The body of the car is given a static charge. The paint is given the opposite charge as it is sprayed on. The paint particles are attracted to the car body and spread out evenly, making for a very smooth paint job.