This short film, which was an ABC after-school TV special (geared to teens) in 1981, does a really good job of explaining why, and how.
It's about a high school history class, and their teacher, who decides to teach them how a political group can manipulate public thought, to the point where they lose their grasp of basic ethics.
And it's based on a true story; it really happened. Ron Jones was the real name of the teacher, and he taught at Cubberly High School in Palo Alto, California. He ran the week-long classroom experiment in 1967, and later published a short story based on the results in 1976.