If one brings two 1lb pieces of uranium 235 slowly together, as they reach criticality, do they start to visibly glow? At what distance? As they get closer, how bright do they get? Like glowing coals? Or too bright look at? If they were slammed together with hands, how big would the explosion be? If the first reaction in Chicago in 1942 had run away, what was the worst damage that might have resulted? The whole campus destroyed, or just the lab? If one could look into the heart of a nuclear reactor, what would one see?