What is the safest way to hook up car battery jump cables and why?
Many recommended procedures such as this one call for connecting (+) to (+), then (-) on the "good" battery to a metal component in the "dead" car engine.  The negative terminal of the battery is grounded to the metal components, so we may think of the last connection as (-) on the dead car.  If a spark were to form when the circuit is established on the last connection, it would occur at the metal component and not at the terminal where it could interact with electrolytically generated hydrogen or oxygen gas.