A simple example would be of when a child hears grown ups talking about an occurrence, say an automobile accident or a plane crash.  The adult may be very emotional or dramatic about it.  Since children learn many of their beliefs from adults, they may adopt the fear they heard in the voices or saw in the actions.  The child then grows up to have an anxiety about driving in cars, or flying in planes, and doesn't even remember why.  Sometimes just remembering the scenario and realizing why the fear exists is enough to reduce it dramatically.