what is a treasure?

We're discussing theme in class right now, and the kids are really struggling. It's just this side of difficult for them--it's quite an abstract concept, and they're still concrete in most of their thinking. This tendency was most evident when we were discussing the "treasure" in Walter Dean Myer's The Treasure of Lemon Brown. In that story, there is a concrete object, the harmonica, referred to as a treasure. It was extremely difficult for them to let go of the harmonica and realign their thinking to more abstract things such as memories or familial love. But, in the end I think some light bulbs went on. I'm thinking that tomorrow I will read a few of Aesop's fables to the kids and have them puzzle out the theme of a couple in succession. That should give us some practice, and it might be fun as well!