How did people know which plants were poisonous before modern science?

People learned which plants were poisonous the hard way - through trial and error, observation, and sadly, through accidental deaths in their communities. Elders passed down knowledge orally about which berries, roots, or leaves made people sick or killed them, and this accumulated wisdom got embedded in folklore and traditional medicine practices across cultures. Healers and herbalists built their expertise by carefully testing plants in small doses and watching what happened, noticing which ones could cure and which ones caused harm. Some cultures also observed what animals would and wouldn't eat, using that as a clue to what was safe for humans.