Why did so many people go to work in factories during WWII?

To understand why this happened, just think about the fact
that WWII was the most mechanized war in history up to that point.  Think about all the
airplanes used in that war -- the first war where airplanes were extremely important. 
Think of all the ships that were needed to carry supplies from the US to Europe as well
as all the way across the Pacific Ocean, eventually almost all the way to
Japan.

Then think about all the bombs and the guns and the
bullets and the uniforms and the food.  All of that stuff had to be made by someone and
those someones had to do the work in factories.

So the
answer to this is that so many people went to work in factories because so many
manufactured goods were needed to fight this war.