We know why the Berlin Wall was built: to stop people from moving away from the Soviet sector in Berlin. But obviously the communists didn't want to come right out and say that. So what did they tell people? Hint: they didn't just try to pretend that the wall was invisible, or had just appeared mysteriously one night.

A lot of the explanations had to do with fascists and the other allies' supposedly lax de-Nazification of their sector. Basically, "There are still so many Nazis on the loose that we have to build a wall to keep them out…erm, in. In your section, not in ours" (source).