"Poisonous" refers to substances that can cause harm or illness when ingested, inhaled, or absorbed by living organisms. These substances can disrupt normal biological functions, leading to symptoms ranging from mild discomfort to severe health issues or even death. Common examples of poisonous substances include certain plants, chemicals, and animal toxins. In nature, many organisms have developed poisonous traits as a defense mechanism against predators. For instance, the poison dart frog produces toxins in its skin, while the deadly nightshade plant contains compounds that can be harmful if consumed. Understanding what is poisonous is crucial for safety in both everyday life and in the wild.