Picture books are aimed at young children. Many are written with vocabulary a child can understand but not necessarily read. For this reason, picture books tend to have two functions in the lives of children: they are first read to young children by adults, and then children read them themselves once they begin learning to read. Board books are picture books printed on sturdy cardboard called paperboard, for young children who tend to be less careful with books. Paperboard is used for both the cover and the interior pages. The pages are specially folded and bound together.