Here's why:
* Alliteration is the repetition of the same consonant sound at the beginning of words that are close together. In this sentence, 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"). This sentence doesn't contain any words that directly imitate a sound.