“When he put the old-fashioned mechanical toy on her palm, she stopped breathing. It was a tiny representation of an atom, complete with colored ball bearings standing in for neutrons, protons, and on the outside, arranged on arcs of fine wire, electrons. Turning the key on the side made the electrons move, what she’d thought were ball bearings actually finely crafted spheres of glass that sparked with color. A brilliant, thoughtful, wonderful gift for a physics major.“Why magnesium?” she asked, identifying the atomic number of the light metal. His hand on her jaw, his mouth on her own. “Because it’s beautifully explosive, just like my X.”