At the Trinity test site, as soon as he saw the blast, Fermi started ripping up little pieces of paper. At the very moment the shockwave from the blast arrived, it blew Fermi's paper scraps away. Fermi then took out a slide rule and said, "Ah, 12,000 tons of TNT." From the papers and position, he could calculate the explosive power of this new weapon.
Physicist Gino Segre, relating a story he heard from his Nobel Prize winning uncle Emilio Segre, about working with Fermi on the atomic bomb at Los Alamos, in an interview in today's New York Times.