In John Watson's experiment on classical conditioning, the white rat was used as a conditioned stimulus to condition Albert. At the beginning of the experiment, the white rat was a neutral stimulus, meaning that Albert didn't show any sign of fear or reaction when presented with the white rat. During the experiment, the white rat was presented to Albert, and its presence was always paired with an unpleasant loud noise (unconditioned stimulus) which elicited Albert's fear and cry (unconditioned response). At the end of the experiment, the white rat itself started eliciting Albert's fear and cry (conditioned response), so the white rat was used as a conditioned stimulus.
