Poisonous means capable of causing harm, illness, or death if you eat it, touch it, or breathe it in. Poisonous mushrooms can make you seriously sick if you eat them, which is why people should never eat wild mushrooms unless an expert identifies them as safe. Some animals are poisonous, meaning their bodies contain toxins that can harm you if you eat or touch them. Poison ivy is a poisonous plant: touching its leaves causes an itchy, painful rash.

Rat poison kills rodents. Carbon monoxide is a poisonous gas that's dangerous to breathe. Even some fish, like pufferfish, contain poison in their organs.

People sometimes confuse poisonous with venomous, but there's a difference: poisonous things harm you when you eat them or touch them, while venomous creatures actively inject their toxins through bites or stings. A poisonous frog hurts you if you touch it. A venomous snake hurts you when it bites you.

Scientists and poison control experts keep careful lists of poisonous substances so people know what to avoid. Understanding what's poisonous helps keep people and animals safe.