There is a level at which everything in the outside world looks alike to a computer program. Unlike the computer memory area allocated for the use of a program, which is a sort of closed system, the outside world is full of moving, changing collections of information, which we can think of as flowing streams. You can dip your hands into a stream of water, take a drink, dip again, and drink again, but the stream is different the second time from the first, and so you are not drinking exactly the same water. Likewise, information gathered from a stream twice in the same way will often differ, whereas assigning a value twice from something that is not a stream almost always yields identical results (with a few exceptions such as rand and ...

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