An unconditioned stimulus is anything, which can evoke a response without prior learning or conditioning. For example , when a dog eats some food it causes his mouth to salivate. Therefore the food is an unconditioned stimulus, because it causes a reflex response (salivation) automatically and without the dog having to learn how to salivate.
Unconditioned Stimulus – This causes an automatic reflex response. Conditioned Stimulus (bell (CS) The conditioned stimulus is created by learning, and therefore does not create a response without prior conditioning.