It depends on the type of water cooler, but you can probably
find the capacity at the bottom of the cooler.
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Because the water is cooler the hot touch that you have causes
the water to be cooler than what is is actually.
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A water cooler usually refers to a water dispenser that chills
the water. Some people also refer to a water fountain (one of those
things you drink water from in public buildings or parks) as a
water cooler as well.