A phone book analogy is often used here. You have a list of people's names in a phone book. The root would be analogous to the first letter of their last names. The intermediate would be subgroups of that, so, for example, Smith to Styles (but not their individual values), and the leaf would be the individual entries (each person's name (and telephone number)).
(Changed my avatar from a Husky to Carl from the movie UP! for the new year. :-))
The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge. - Stephen Hawking