What is the "Kobald Box" as noted in the nonfiction book, One Child?

Torey invents the Kobold box as a way to encourage the children to behave and learn to compliment other children who act nicely. She tells the students that the Kobold lives in their classroom and watches to see if the children do kind things. Torey expands the Kobold box by encouraging the children to leave notes in the box whenever someone does something nice. The Kobold box becomes a favorite of the children and they eagerly look forward to reading the notes at the end of each day.