Here's why:
* Alliteration is the repetition of the same consonant sound at the beginning of words that are close together. In this case, the "s" sound is repeated in "Susie," "saw," and "seashells."
* Onomatopoeia is the use of words that imitate the sounds they describe (like "buzz," "meow," or "crash").
Let me know if you have any other wordplay questions!