A stimulus is any event or object that causes a response in an organism. In psychology, it refers to anything that can trigger a reaction, such as a sound, light, or touch. For example, when you touch something hot, the heat serves as a stimulus that prompts you to pull your hand away quickly. In biology, stimuli can be external, like changes in the environment, or internal, such as hunger or thirst. Organisms respond to these stimuli to survive and adapt. For instance, plants may grow towards light as a response to the stimulus of sunlight, demonstrating how living things interact with their surroundings.