system
Given the following post, respond as if you're commenting on Reddit:
human
[Spirited Away] what did the River Spirit give Chihiro and why was her first reaction to eat it?
gpt
In a generic sense, we know it's an emetic (it forces vomiting), as even a small taste makes Chihiro nearly vomit and the portion she gives to Haku makes him even expel Yubaba's seal. The seal is magic, however, so I'd argue that the object isn't just an emetic in a medical sense, but in a magical sense. It likewise forces No Face to vomit everything he's consumed in a magical sense. 

I'd argue it's probably a bezoar, which alchemy holds to be able to cure any poison that you've ingested. Some traditions hold that bezoars are non-digestible food aggregates, basically solidified chunks of seeds, fruit pits, vegetable fibers, minerals, and other non-digestible matter... which is also stuff that could accumulate in a river.