If a child covers her ears when seeing fireworks, before hearing the sound of them, seeing the fireworks would be Conditioned stimulus.
Conditioned stimulus is a stimulus that can produce a conditioned response. It produced a reaction only after the subject has learned to associate it with a given outcome. For example- a dog attacked you at a particular place, so whenever you come across a dog you start fearing. So in this case the child covers hers ears as she already had a previous experience of similar kind.
Unconditioned stimulus is a stimulus that produces automatic response. For example- When you smell your favorite food you immediately start feeling hungry.
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