I've been learning about radios for a while, and this article explained one of the key questions I had: why can't you turn on and off some single frequency waveform faster to transmit data faster? (answer: changing amplitude messes with the spectrum and makes it no longer a single frequency...)

I tend to prefer these visual and intuitive explanations to the mathematically based ones usually given in lectures. The "open capacitor" example was something I hadn't thought of before.