system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Can someone explain to me why they don’t recommend putting car jumper cables on the working battery black terminal to the dead battery black terminal but rather to an unpainted metal surface on the donor car?

Also, how does electricity flow from one car to the other if there isn’t a loop?
gpt
Non-perfectly sealed car batteries will slowly leak flammable gas, and an active spark would light it on fire. So the proper procedure has you connecting the last time (which is most likely to produce sparks) far away from this potential fire source.

Cars are wired so that 'ground' connects to the negative terminal, and 'ground' is basically the metal chassis of the car. So connecting to the metal frame of the car *will* connect the batteries in a loop.