The Berenstain Bear's Big Book of Science and Nature.
It is because of this book that my kids know their months and seasons. It is because of this book that they can tell you the different characteristics of plants and animals.
It is why my daughter knows that frogs and toads are types of amphibians.
It is why my son knows plants make oxygen for us to breathe.
It is why they understand the basic types of machines, and how to build one.
And today, it is why they understand the three kinds of matter, how something moves from one state to another, and why helium balloons stay up in the air.
I'd love to take credit for their education, but pretty much everything they know, we owe to Mama and Papa Berenstain.