Coolant

Why?   Engine coolant (a mixture of antifreeze and water) does more than just lower freezing points and increase boiling points. It lubricates the water pump, inhibits the formation of rust and prevents the buildup of corrosion in the radiator and heater cores. These areas are particularly sensitive to a process in which solids drop out of suspension and solidify in the areas of greatest temperature drop. Over time, the additive packages, which prevent foaming, corrosion, oxidation and provide boil and freeze protection, are depleted. The coolant becomes corrosive and mildly abrasive and can damage the block, water pump seal, heater core and radiator.

The heat generated by your engine is sufficient to cause it to literally melt down in the event of cooling system failure. A compromised cooling system can destroy the engine, requiring its replacement.