In classical conditioning, there are stimuli that can produce responses all by themselves and without any prior learning. These types of stimuli are called unconditioned stimuli (US or UCS) and they evoke unconditioned responses (UR or UCR), or responses that are completely natural and occur without an organism going through any prior learning. For example, if you smell a lemon, it might get a sour taste in your mouth and you may salivate. This may occur from the time you are born and can occur without you ever having tasted a lemon before. The salivation and sour taste would be unconditioned responses.