The insperation for the Junkbots project came from reading the seminal book Junkbots, Bugbots, and Bots on Wheels by Dave Hrynkiw and Mark W Tilden. In this book they discussed turning electrical 'junk' into some very interesting robots - and sneaking in some very interesting engineering challenges and principles.
A sample chapter can be found here looking at producing a solar powered motored 'bug'.
The robots built within the JunkBots project are simpler but the principles are the same. If you are interested in this area this book is a must read.