This book of almost 2.50 puzzles begins where our first book. Mad About Physics: Braintwisters, Paradoxes, and Curiosities (2001) ended—with the physics of the late nineteenth and early twentieth centuries. The Michelson-Morley experiment of 1887, the challenges posed by atomic spectra and blackbody radiation, the unexpected discoveries of X-rays in 1895, radioactivity in 1896, and the electron in 1897 all loosened the protective belt of ad hoc hypotheses around the mechanistic physics the nineteenth century had so laboriously built. Anomalies and paradoxes abounded, ultimately necessitating a radical rethinking of the very foundations of physics and culminating in the theory of relativity and quantum mechanics. Numerous applications of these new and strange concepts followed very quickly as atomic and nuclear physics led to semiconductor devices on the small scale and nuclear energy on the large scale. Therefore we have developed a whole new set of challenges to tickle the minds of our scientifically literate readers, from science students to engineers to professionals in the sciences.