Firsthand historical account, Los Alamos Science special issue, 125–130. The Spark and The Method sections, pp. 126–127, describe Ulam’s suggestion, von Neumann’s development, and simulated neutron histories. Author printed as N. Metropolis; full name verified by the original 1949 JASA joint-paper publisher record doi.org/10.1080/01621459.1949.10483310 .