This letter made me think, so I polled my less mechanically inclined friends and was surprised to find out how many people were in the dark on the subject of coolant:

Hi Matthew, I was just reading your maintenance guide at About.com and I have a couple of questions about my car's coolant. I drive a 94 Volkwagen GTI, and when I bought it there seemed to be only water in the coolant bottle. Is it possible to have run on just water without overheating? Or is this more likely a mix of water and coolant? In your guide you state that using tap water is harmful for the engine so I'm just wondering what the previous owner had in the cooling system. If it is indeed just distilled water and no coolant in the cooling system, could that have caused harm? I think I will flush out the cooling system and put in the OEM recommended coolant, is there any further steps you'd advise?

Thanks in advance for any help, Tim

The difference between running the proper mix of coolant or just water is huge. It's not enough for your radiator to be wet on the inside, coolant has properties that perform a number of different functions while it swooshes through your engine. The most obvious difference is the freezing point, if you live in a cold climate you'll crack your engine block if you have only water in there. But coolant also has a much higher boiling point than water, and can therefore hold a lot more heat without boiling over. Wait, there's more. The compounds in coolant fight corrosion in the radiator and other components it comes in contact with. Running water alone isn't going to stop the process, and can lead to premature failure of parts of your cooling system. Worse, tap water has its own bag of unwanted chemicals, and running your car on that can cause even more corrosion of parts. The bottom line? You must run coolant in your system no matter where you live. This is non-negotiable. If you suspect your car is without coolant, it's a good time to do a flush and fill of the system. It's not worth taking chances.

http://autorepair.about.com/b/2008/09/12/coolant-or-water.htm