I just read Six Easy Pieces by Richard Feynman which is a selection of the six easiest lectures he gave in a two year course to Cal Tech freshman physics majors. It was a fun book though I can't claim I didn't get lost a couple of times when he was talking about positrons and particles and the like. However, the first piece Atoms in Motion was quite clear, enough so that I am considering having my 8th grader read it because it deals with the subject that both he and Paddy are studying right now for science.... states of matter and how heat changes the motion of molecules. I found Atoms in Motion in pdf online in case anyone else happens to get into these sorts of things.
Paddy and I did a simple but fun experiment to show the heated water molecules' faster movement--here's a similar lesson plan Here's another experiment of the same type but to do it I'd have to figure out a substitute for baby food jars ;-).