How did Helen learn the word "water"?

At the very beginning of the book, Keller tells us that the word "water," which was one of the few words she had learned before her illness, remained with her even after she went deaf and blind. She continued to make some form of this word until she learned how to spell it out, indicating that this was a fundamental concept for her and that she had some existing understanding of it as a word.
Later, Helen's teacher named Ms. Sullivan begins to teach Helen to spell out words by spelling letters into her hand and then putting Helen's hand in contact with the object the word describes. This works to an extent, but it is only when she learns the word "water" that Helen has a true breakthrough.
Ms. Sullivan takes her outside into the warm sunshine, and the two walk down to a well house, where water is jetting from a spout in a "cool stream." Placing Helen's hand under this stream, Ms. Sullivan begins to spell the word "water" into Helen's other hand, over and over. Suddenly, Helen realizes that the water flowing over her hand is represented by the symbols being written into her other hand, and the previously lifeless symbols suddenly become real to her.