Helen Keller relies on allusion to explain to the reader the extraordinary importance of Miss Sullivan's entry into her life. She compares Miss Sullivan's arrival to the Israelites crossing the Red Sea to escape from slavery in Egypt to freedom in the Promised Land. She uses this reference because it would be understood by most readers at the time, and it helps communicate the magnitude of Miss Sullivan's coming into and changing her life.
Keller again uses a biblical allusion when she writes, after she makes the connection between actual water and Miss Sullivan writing "water" in her hand, that
words ... were to make the world blossom for me, "like Aaron's rod, with flowers."
Aaron's rod miraculously bloomed with flowers and bore almonds showing that his tribe, the Levites, were meant to be God's priests. Words likewise become a fruitful rod for Helen. Aaron's rod was a source of authority, so Keller is also using this allusion as a metaphor or comparison: words blooming in her life...
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