My first radio project was a crystal set. I was 8 or 9 when I built my first one. (I divide up my childhood in segments, the first one being our house on Desoto Drive in Miami Springs, where we lived until 1954, and the second when we moved to the house on Falcon Avenue. I have clear memories of building crystal sets on Falcon Avenue, but none earlier.) It may have been Cub Scouts where these things first appeared in my life - Mom was a Den mother - but my dad was the major instigator. He had built them when he was a growing up, and helped me collect the parts and build my first couple of them. I found it wondrous to put some wire on a board, string more up out the window as an antenna, connect some earphones, and hear radio stations.
The "crystal" which is the core of the radio, "detects" radio signals, and was called a "crystal detector." It changed the alternating current that came down the antenna wire into direct current, which not only was able to carry the "information" in the signal, but also to power the radio itself, for it had no battery. The "crystal" was a crude semi-conductor, and my first sets actually had the little piece of crystal rock encased in a tiny metal bowl connected to the antenna. The apparatus had a piece of wire (the "cat's whisker"), one end of which you applied physically to the rock, and the other of which went to the next component. Later I graduated to a kit that had an actual "diode," which encased the bit of crystal in a tiny tube with a wire coming out of each end. Little did I know that this would be to transistors and then computer chips what my roller skates were to the automobiles I now drive. (There's a good article on crystal radios at Wikipedia.)
Lately I have come upon a website that is devloted to crystal sets, "The Xtal Set Society." Uncotrolled nostalgia, yes, but hand me my pliers. And, Aidan, save your oatmeal boxes.
(Ken, didn't you get into this too about the same time?)