Why you must disconnect the negative terminal of your car battery first ?

Whenever you work on a car battery, always disconnect the negative terminal first and not the positive and when re-connecting, connect the positive first.

The reason is pretty simple: If you accidentally touch the engine-bay or any other grounded metal part of the car with the positive, nothing will happen. But if the negative terminal is still connected, and if you happen to touch any metal, it would result in disastrous consequences(which I don’t need to explain even to a high-school grader) since the battery will be conducting several hundred amperes.