Disconnecting the positive cable first on a car battery can lead to sparks due to the risk of creating a short circuit if a metal tool contacts the exposed positive terminal and the car's metal frame. This is because the negative terminal is connected to the vehicle's chassis, which acts as a ground. If the positive terminal is disconnected first, any accidental contact can cause high current to flow through the tool, resulting in sparks and potential damage to the vehicle's electrical system. Removing the negative terminal first eliminates this risk, as it disconnects the ground connection. Therefore, it's crucial to follow the proper procedure to ensure safety and prevent damage.