Ivan Pavlov, in 1927, began working with learning through "classical conditioning." Initially the dog only salivated when it was eating. Later Pavlov noticed the dog salivated when he carried the food into the room. He become curious as to why this change had taken place. He thought there were both learned and unlearned components to the dog's behavior. He began experimenting with different stimuli, and if he rang a bell immediately before giving food to the dog, eventually the dog would salivate merely in response to the sound of the bell. He generated terminology to describe his observations. An unconditioned stimulus (UCS) such as food, generates and instinctual reflexive, unlearned behavior, such as salivation when eating. The salivation was called an unconditioned response (UCR) because it was not learned. The bell, formerly a neutral sound to the dog, become a conditioned learned stimulus (CLS) and the salivation a conditioned response (CR).