how does tap water stay cold?!
Lower than a few feet down, the earth is fairly cold, it is from being contained underground. You will notice that the cold water is not as cold as it is in the winter, when it is considerably colder and subject to freezing. Interesting note, before refrigeration, people use to keep their food in cellars to keep them chilled.
its not always cold....but i guess cuz its down under ground (no sun hitting it)