The article on elementary particles explains that there are two types of particles: bosons and fermions. This explains why Millie’s adventures begin at the border between the territory of the bosons, Bosonville, and that of the fermions, Fermioncity.

In Millie’s adventures, fermions live in Fermioncity, and bosons in Bosonville.

Electrons are fermions, but so are quarks in Book 5, neutrinos in Book 6, and antiparticles in Book 7.

In Book 3, Millie goes to the playground in Bosonville to play with photons, which are bosons. Millie discovers the existence of W bosons in Book 7 and of the invisible infrared photon in Book 10.

In Book 8, Millie learns about the differences between these two types of particles.