Final answer:
Initially, the bell in Pavlov's experiments was a neutral stimulus, not eliciting any response, and after conditioning, it became a conditioned stimulus, then causing salivation on its own.
Step-by-step explanation:
In Pavlov's original experiments, the bell served as a neutral stimulus initially. A neutral stimulus is one that does not naturally elicit a response from an organism. During the conditioning process, Pavlov presented the bell sound prior to giving the dogs meat powder. The meat powder was the unconditioned stimulus (UCS), which naturally caused the dogs to salivate, this salivation being the unconditioned response (UCR). After several pairings of the bell with the meat powder, the bell sound came to elicit salivation on its own. Once this association was established, the bell became a conditioned stimulus (CS), and the drooling in response to the bell became a conditioned response (CR).