After eating at In-and-Out Burger, you become really sick that night and continuously throw up.  The next day, the smell of In-and-Out makes you nauseas.  This is an example:

This is an example of classical conditioning, a type of learning process that was first described by Ivan Pavlov. The steps are as follows:

1. Before Conditioning: Initially, the smell of In-and-Out Burger is a neutral stimulus, it does not cause a reaction.

2. During Conditioning: You ate at In-and-Out Burger and then became sick. This pairs the neutral stimulus (the smell of In-and-Out Burger) with an unconditioned stimulus (the food that made you sick).

3. After Conditioning: The smell of In-and-Out Burger alone now causes you to feel nauseous. It has become a conditioned stimulus, causing a conditioned response (feeling nauseous).

So, in this case, the sickness acted as an unconditioned stimulus which led to an unconditioned response (throwing up). After association between the unconditioned stimulus (sickness) and a neutral stimulus (smell of In-and-Out Burger), the neutral stimulus became a conditioned stimulus causing a conditioned response (feeling nauseous at the smell of In-and-Out Burger).