This chapter is the heart of this book: A paragraph-by-paragraph analysis of the physics behind the quantitative part of the Frisch–Peierls memorandum. Topics include the nature of uranium isotopes, slow and fast-neutron chain reactions and their timescales, why a bomb requires a fast-neutron chan reaction, critical mass, likely energy yield, the danger of weapon predetonation, how uranium enrichment could be realized, and radiation effects. Their document still makes for a compact primer on the physics of nuclear weapons.