Book 8 in which Millie wants to give the particles a sleeping lesson Millie micro nano pico Book 8 is for all the children who asked Millie to find out more about the inhabitants of Bosonville and Fermioncity. Millie knows that photons are bosons and electrons are fermions. In this adventure, she discovers that you can put as many photons as you like on the same energy level. On the other hand, only a pair of electrons with opposite spin can occupy the same energy level. Like all fermions, electrons obey the Pauli exclusion principle.

Millie also learns about the nature of X-rays and convinces herself that elementary particles are unbreakable.