How does the author Rick Riordan in the book:The Lightning Thief, use allusion to move the plot forward

Riordan absolutely loves making allusions to ancient mythology. In the Lightning Thief for example, when fighting Medusa, Percy avoids looking at her by using a garden sphere. This is an allusion to the myths, when Perseus avoided looking at Medusa by using his own shield like a mirror. That's a simple example. Let me know if you need more.