What is the purpose of the anecdote with which david foster wallace opens the speech? what is the effect of his returning to the line "this is water" again near the end of the speech and repeating it there?

The author of “This is Water”, David Foster Wallace, uses the anecdote of the wise old fish and the younger fish to show us the importance of being aware of our surroundings. By utilizing this anecdote, he relates the importance of our awareness to the importance of water to fish.

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