What event helps Helen recognize the meaning of water?

From the text:

In a previous letter I think I wrote you that "mug" and "milk" had given Helen more trouble than all the rest. She confused the nouns with the verb "drink." She didn't know the word for "drink," but went through the pantomime of drinking whenever she spelled "mug" or "milk." This morning, while she was washing, she wanted to know the name for "water." When she wants to know the name of anything, she points to it and pats my hand. I spelled "w-a-t-e-r" and thought no more about it until after breakfast. Then it occurred to me that with the help of this new word I might succeed in straightening out the "mug-milk" difficulty. We went out to the pump-house, and I made Helen hold her mug under the spout while I pumped. As the cold water gushed forth, filling the mug, I spelled "w-a-t-e-r" in Helen's free hand. The word coming so close upon the sensation of cold water rushing over her hand seemed to startle her. She dropped the mug and stood as one transfixed. A new light came into her face. She spelled "water" several times.