system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 Why can you jumpstart a car battery with the black cable on the negative pin on the battery or the car frame? Doesn’t the electricity flow negative to positive?
gpt
The car frame is joined to the negative of the battery.  It acts as an "earth" if anything goes wrong and shorts out to the frame in the car.  You wouldn't want to have the metal door handles, etc. be accidentally made live, or floating (then you would get a lot of static on them from the road movement building up a charge).  So you connect them to negative.

But when you jumpstart a car, it's recommended to make sure that you keep the final connection AWAY from the battery.

Lead acid batteries release hydrogen when being charged.  Hydrogen is flammable.  If you complete the circuit to the battery, which charges the battery, and that connection sparks when it makes contact, and the battery is kicking out hydrogen, you could in theory start a fire.  It's unlikely but possible.

So you instead complete the final connection (the negative to the second car) to the actual chassis of the car, away from the battery.  Because the chassis and the negative are joined together, it's electrically exactly the same, but now the spark is away from the battery.