This is a 20-volume set of Gakken's "Naze Naze Best Kodomo Zukan" (Why Why Best Children's Encyclopedia). This encyclopedia clearly answers children's "Why?" and "How come?" questions. It also contains many illustrations, making the content enjoyable for learning. In our home, we often used it for reading aloud to our child before bedtime. Even adults can find many "Oh, really?" moments, allowing parents and children to enjoy reading together. It's an encyclopedia series that was very helpful in sparking children's curiosity. *All covers have been lightly cleaned and disinfected with alcohol. *There are brown spots on one page after turning it. (Not on all volumes, but many.) Please check the images. I believe the covers and the inside are in relatively clean condition. *As far as I could check, there are no writings, but there is a possibility of oversight. I would like to offer this to someone who understands that it is a used item that a child has used. *Those who are particular about the condition, please refrain from purchasing. *All 20 volumes are sold together (not sold individually).