Paul Ehrenfest grew up in a middle-class Jewish family in Vienna. Tatiana Afanassjewa came from a wealthy family in St Petersburg. Their love of science brought them together at the beginning of the twentieth century and led them to Leiden in the Netherlands. There, the ebullient Ehrenfest built up an enormous international network of mostly physicists. Afanassjewa worked -- inevitably -- mainly at home, among the children, on the theory of heat, and thought about the didactics of geometry and how to 'teach children to think'. And as Europe grew darker and darker, the 'bright' Russian house that Afanassjewa had designed blossomed into an oasis for thinkers from all over the world. The list of signatures on the wall of the guest room includes the names of sixteen Nobel Prize winners, including Niels Bohr and, of course, Albert Einstein, Ehrenfest's best friend. Over the past few years, Margriet van der Heijden has delved into the archives to tell the story of Ehrenfest and Afanassjewa and their microcosm, which fell apart when Hitler came to power in 1933. While on the run in England, Einstein heard that Ehrenfest had taken his own life. Afanassjewa had to survive without her professor, who, while "dancing in front of the blackboard", had made physics enchanting. Van der Heijden tells their story using many new documents from the Ehrenfest Family Archive and highlighting not only Ehrenfest's contributions to physics, but especially also those of Afanassjewa whose work on thermodynamics, dimensional analysis and the didactics of geometry has previously gotten less attention.