When you rub a balloon with a wool cloth, the balloon captures electrons from the wool, leaving the balloon with a negative charge and the wool with a positive charge. If both balloons are rubbed with wool, they will both have same charge (negative) and therefore will repel each other. When you spray water near the balloons, the water droplets carry the charges away from the balloons, leaving them uncharged. As the balloons become neutral, they will fall back together.
Another way to neutralise the charges on the balloons is to touch them with a damp or moist hand.
Doing static electricity experiments in damp or humid weather can sometimes have confusing or unexpected results! Moisture in the air acts just like the spray of water in this demonstration and carries charges away from objects. This makes it harder to build up charges in your experiments.
