Talk to the boy located near the conveyor belts in the mines to learn he wants some Eau de Mouche. Afterwards, go talk to the researcher with a weird hat who will offer to trade Eau de Moche for his favorite snack. Go back to the boy and he will tell you that the messenger girl running around the village has the snack. Talk to her to get the Mountain Apple and give it to the researcher for the Eau de Moche, which you can finally give to the boy in return for a Fire Fruit.