Book 5 in which Millie wishes she were more attractive and wonders about quarks is about electric charge and in particular the elementary charged particles called quarks. The protons and neutrons inside the atomic nucleus are made of quarks.

Millie wants to know how to tell the difference between a neutron and a proton. She discovers that only protons are charged and that their positive charge is the opposite of the negative charge of an electron.

In this adventure Millie learns about the attraction between opposite charges. She also finds out that masses are attracted because of gravity, and that there are similarities between electric and gravitational fields.