Objects usually have equal numbers of positive and negative charges, but it isn't too hard to temporarily create an imbalance. One way scientists can create an imbalance is with a machine called a Van de Graaff (you left this answer blank) generator. It creates a large static charge by placing electrons on a metal dome using a motor and a big rubber band. Since like charges repel (you left this answer blank), the electrons push away from each other as they collect on the dome. Eventually, too many electrons are placed on the dome and they leap off, creating a spark that looks like a bolt of lightning.