I'm curious to what makes something qualify as a ground? I do some electrical work on my car. And the way I understand it, is that the chassis of the car is used as ground because it's such a big metal piece that the chassis doesn't build up enough charges to be considered charged. Therefore it's a 'zero potential'.
But how big does the metal object have to be, compared to the (e.g.) battery it's connected to. In other words, what makes the metal object qualify as a ground?
The reason for asking this question is because in many instructions I am told to "connect the other wire to a ground", and I keep asking myself: Well, what is a ground, and what is not?