Rub each balloon on the sweater, so that they each have negative charges on them. Place one balloon as far away from the sweater as possible on the right. Then bring the other balloon close to that balloon. What happens and why?
When the first balloon is rubbed against the sweater, it gains extra electrons from the sweater and becomes negatively charged. As a result, it repels the negative charges in the sweater and attracts positive charges to its surface. So, the first balloon becomes negatively charged.
When the second balloon is rubbed against the sweater, it also gains extra electrons from the sweater and becomes negatively charged, just like the first balloon.
Now, if we bring the two negatively charged balloons close to each other, they will repel each other. This is because like charges (negative charges in this case) repel each other due to the electrostatic force. The balloons will push each other away, trying to move as far from each other as possible.
This repulsion occurs because both balloons acquired a similar negative charge by rubbing against the sweater. Thus, they have the same charge and repel each other in an effort to minimize their interaction.