Book 6 in which Millie meets two neutrinos and watches them race to the Moon and back is about the elementary particles called neutrinos. Neutrinos are everywhere, but are very hard to detect because they don’t interact much with the other particles. Their mass is extremely small, they have no charge, and they travel nearly as fast as light.

In this adventure Millie realises just how fast light travels, when Lucille, the photon, and two neutrinos race each other to the moon and back. She finds out that photons have no mass and that they always travel at the same speed. Einstein’s special theory of relativity is based on the assumption that the speed of light, just under 300 million meters per second, is constant.