The best picture books work on different levels for different ages.

谷物不仅仅是一种商品，它是文明的基础。

The decline of violence may be the most significant and least-appreciated development in the history of our species.

"The best children's books are those that respect the intelligence of the child."

解释：大旱之年收购船只，大水之年抛售船只并收购车辆，待到大旱之年再抛售车辆。