For several years, I taught a general education course on the Manhattan Project for students majoring in the arts and humanities who needed a physical science credit as a condition of their graduation requirements. As might be imagined, the challenge in teaching this course was to find a balance between quantitative and qualitative content. A technical point of particular importance was to find a way to describe how fundamental-physics considerations restrict the number of possible candidate "weaponizability" isotopes to just a handful, of which only uranium-235 (U-235) and plutonium-239 (Pu-239) were practical for developing a first-generation nuclear weapon. After doing so, I could go on to describe how Manhattan Project engineers developed two weapons: a U-235 bomb based on isolating that isotope from its much more populous U-238 sister isotope, and a plutonium bomb based on synthesizing that element by neutron bombardment of natural uranium in a nuclear reactor.