After signing on to work at the bathhouse, Yubaba gives Chihiro a particularly unpleasant job to do, bathing a smelly, sludge-oozing guest who turns out to be the spirit of a river polluted by garbage.
However, Chihiro is insightful enough to realise what the problem is and with the help of the rest of the staff she frees him from the pollution. In return, the river spirit gives her some medicine described as an "emetic dumpling". Its effect is to make anyone who eats it violently sick. Although she is later able to use the medicine to help No-Face and Haku, it seems an unusual gift.
Is there an in-universe explanation why the river spirit gave her the emetic dumpling, instead of a more ordinary reward? Chihiro seemed to think (with some justification) that feeding it to her parents might turn them back into humans, but it seems unlikely (apart from telepathy) that the river spirit would be able to know about her parents predicament.