Most cooling systems in todays vehicles utilise a glycol based fluid to cool the engine and components called "coolant". Despite the possibility of being hazardous to your person and vehicle paint, this fluid is actually an integral part to the operation of the vehicle engine. No coolant means no cooling for an engine that can cause catastrophic engine failure (unless you have an old VW thats air cooled!!)
The problem with coolant, is not all coolants are created equal. Coolants, regardless of their colour and brand may differ with different chemical make up and additives that if mixed have been known to turn gelatinous (Jelly like) increase viscosity and overheat the engine in seconds!
We have seen this first hand! Replacing complete short and long motors in engines from this! I have told our customers and my students when I taught cert III and IV Automotive, that it is always advisable to 'top up' with distilled water only. More than a couple of cups may cause the mixture to thin out, increasing the likely hood for a lower boiling point and the introduction of corrosion causing water pump seal failure etc. In addition, more than a couple of cups of water to top up may indicate coolant system boiling or loss of coolant through a leak. Either way if it needs more water, get it to a mechanic to have a quick sqiz!
Cheers! Mike
Disclaimer: Please utilise the vehicle owners manual for methods of cooling system check and top up and always ensure that work is carried out by a qualified mechanic ensuring that the cooling system is bled correctly!