Why does a negative battery terminal spark when cables are connected?

A negative battery terminal can spark when cables are connected due to the sudden flow of electrical current as the circuit is completed. If there is a difference in potential or if the battery is under load, connecting the cable can cause a brief surge of electricity, resulting in a spark. Additionally, if there is corrosion or a poor connection, the increased resistance can also contribute to sparking. It's generally safer to connect the positive terminal first, then the negative, to minimize sparking.