In Classical conditioning, the conditioned stimulus was previously a neutral stimulus that eventually becomes to trigger a conditioned responses after becoming associated with the unconditioned stimulus.
Here is an illustration of classic conditioning, the unconditioned stimulus (food) is presented repeatedly just after the presentation of the neutral stimulus (bell). After conditioning, the neutral stimulus alone produces a conditioned response (salivation), thus becoming a conditioned stimulus. Explanation, from this illustration, one salivates whenever it sees food but before the present the food, a bell is rung. Overtime just ringing the bell makes the person to start salivating.
