In the article about the discovery of the electron, we read that the atom is more empty than full. The nucleus is ten thousand times smaller than the atom, and the electrons are very far apart. This is why, when Millie first arrives in the world of particles (book number 1), she feels completely lost in a huge emptiness. Her confusion is so great that she considers giving up her adventure and going back home.

The idea of emptiness within the atom and the huge distance between electrons and the nucleus is revisited in book number 5 when Millie wants to see the quarks, the particles inside the nucleus. Her absurd idea of using a telescope greatly amuses the electrons.