system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do we need to attach one negative cable to bare metal when jumping a car?
gpt
Lead acid car batteries can emit hydrogen gas (hydrogen sulfide typically) under certain conditions which is flammable. Since we are taught to attach the negative last, it usually creates a spark. Attaching the negative to a point of the car away from the battery puts distance between the spark and a potential (albeit rare) ignition. So safety is the reason. Note that even many “sealed” batteries can off-gas.