Book 3 in which Millie meets some photons at the playground is a playful description of the interaction between electrons and photons. At first, Millie is confused by the game that the particles play, but after learning about energy levels, quantisation and in particular the law of conservation of energy, she starts having fun.

The ideas in this adventure are related to the photoelectric effect, which is impossible to understand in terms of classical physics. Einstein was awarded the Nobel prize for his explanation of the effect. This was one of four remarkable discoveries he made in 1905, which included the theory of relativity.

The physics concepts in this story are important for the production of electricity from solar energy, and for the operation of modern electronic devices, like smartphones, computers, and cameras.