More than 20,000 schools, 60,000 teachers and about two million students use the materials.
It was always a risk, using human material like these.
I could have used material that I'd come up with before.
He looked at new ways of using other materials too.
But there was good material used in the old days.
They also use different materials from those common in other parts of the world.
"It seemed like a great idea to use the material again."
I do not own any of the material used in this video.
"The idea is to use the materials that are here now."
"We will also be using material from other news organizations."