Stimuli are any events or changes in the environment that can provoke a response from an organism. They can be physical, such as light, sound, or temperature, or chemical, like the presence of certain substances. Organisms detect stimuli through their sensory organs, which help them interpret and react to their surroundings. In psychology, stimuli are often studied to understand behavior and reactions. For example, in experiments involving Pavlov's dogs, a neutral stimulus, like a bell, was paired with food to elicit a response. This process illustrates how organisms learn to associate certain stimuli with specific outcomes.