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In classical conditioning, an unconditioned stimulus (US or UCS) is any stimulus that can evoke a response without the organism going through any previous learning; the response to the US (the unconditioned response) occurs naturally. 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 lemon, therefore, is a US since it produced the salivation and sour taste (the UR) naturally, without you having any previous experience with lemons.