When using jump leads they say you need to connect the positive (red) terminal first.

I have tried to understand this, why not the negative first? As the negative is invariably the earth, the common ground, even if the cars are insulated from the ground, still, I cannot see why the positive first.

I have imagined it may have to do with "what things you can accidentally touch with the jump lead terminal and cause sparks", but it seems the same risk either way.

any ideas?