Here's why:
* Onomatopoeia is the use of words that imitate the sounds they describe. The word "purr" directly imitates the sound a cat makes.
* Alliteration is the repetition of consonant sounds at the beginning of words. While the phrase has the repeated "p" sound, it's not the primary characteristic that makes the phrase interesting. The word "purr" itself is the focus, not the repetition of sounds.
Therefore, "the cat said purr" is a clear example of onomatopoeia, not alliteration.