My first idea was to make something like a house of cards, with each wooden "card" having puzzle piece type ends so that they locked together.
I immediatly realized the importance of the ratio of the depth of the notch to the thickness of the joining piece. I also found that when cutting very thin pieces of wood, they are strongest when the grain of the wood is longest. For example, a day or so after ripping the wood vertically, I found that some of my pieces were begining to bend, following the curvature of the growth rings from the tree.
In exploring design concepts, I wanted to create something useful, like a shelf that was able to be hung on the wall. In combining the two ideas, I came up with a modular shelving system that locked together in a puzzle piece like fashion.