Coolant is generally a liquid or gas substance used to regulate the temperature of your engine. Liquid coolants are usually half water and half ethylene glycol. Water is used to transfer heat while ethylene glycol is used to lower the freezing point of the liquid. In colder climates, this is extremely troublesome. Interestingly enough, this isn't the earliest time you've heard of ethylene glycol as it is the available ingredient in the classic term "antifreeze". You never want to pour antifreeze or water by themselves into your engine system. They need to be combined to create the decent coolant.
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