I propose to bring before you, in the course of these lectures, the Chemical History of a Candle. There is no better, there is no more open door by which you can enter into the study of natural philosophy than by considering the physical phenomena of a candle.–Michael Faraday, introduction to lecture 1
This is my all time favorite DIY science book. 150 years ago, the great Faraday (and I do mean great; I don't believe there has been an experimental scientist of his ability since) gave a series of lectures for school children at London's Royal Institution. In six lectures he explained many mysteries of chemistry and physics using a wax candle and some very simple props. The text for all six lectures are available for free online. I am still looking for an online edition that contains the drawings, which are pretty important.