Not just sparks. The electrical system of one or both cars can be severely damaged.
Also, you shouldn’t attach the negative cable to the negative terminal on the dead car, but to exposed metal of the car’s chassis (i.e. structure). This is to avoid a spark igniting any leaking fluid from the bad battery. Flaming battery acid = not fun.
Not just sparks. The electrical system of one or both cars can be severely damaged.
Also, you shouldn’t attach the negative cable to the negative terminal on the dead car, but to exposed metal of the car’s chassis (i.e. structure). This is to avoid a spark igniting any leaking fluid from the bad battery. Flaming battery acid = not fun.