The answer to the title question is more than just, ""Money"". After reading this the young rider will be able to say a lot more than that. It starts with the electrical system and stops --logically enough -- with the brakes. In between has come the fuel pump, cylinders, clutch and various other mechanical mysteries. Explaining the gear system could have been the downfall of this effort, but, between pictures and text it is made beautifully clear. The illustrations are black and white drawings by Len Darwin. They are blocky, simple representations of the essential parts and never fail to complement the text. They will not date quickly. This should be useful for the innocently ignorant adult as well as for children who like to know what it is that makes things go.