Adventure Comics #251

In a flashback story, Superboy learns about kryptonite for the first time. His school assignment is Geology, so Pa Kent brings home a \"green glowing rock\" that sickens Superboy near death. Superboy sends his robots out on missions, except for his first obsolete lead-lined robot. Only when the robot steps in front of the deadly kryptonite does Superboy figure out the radiation. So it was sentimentality that saved his life!