A conditioned reflex is a learned response to a stimulus that occurs through repeated association. This concept was famously demonstrated by Ivan Pavlov, a Russian physiologist, who observed that dogs would salivate not only when food was presented but also when they heard a bell that had been paired with feeding. This process is known as classical conditioning. In classical conditioning, an unconditioned stimulus (like food) naturally elicits an unconditioned response (like salivation). When a neutral stimulus (like a bell) is repeatedly paired with the unconditioned stimulus, it becomes a conditioned stimulus, leading to a conditioned response. This illustrates how behaviors can be modified through experience.