A friend of my wife was "trying" to help out and refill the radiator with engine coolant. She found out later that what she did was a BAD thing. I've heard that is true..but how? It flows through a radiator to a pump to holes in the engine to absorb the heat and distribute it to the radiator to be cooled and start the process all over again. As far as I know it doesnt touch any vital parts...so how is mixing anti-freeze a bad thing?