If you strum an electric guitar and let the hertz of the string fall through the range of AM radio the amp will briefly pickup AM radio stations. Not that you can decipher anything but you recognize voices as it travels past the station.
Such naturally occurring diodes are an interesting phenomenon (in this case: the salt in your sweat interacting with the steel of the strings) and were the basis of the very first radio receivers after the 'Coherer' (which is a word that has fallen out of use so far that it registers as a spelling error on my browser!).
Of course, if you want to build a proper AM receiver you will find that it is quite hard ;)
Same with oscillators and amplifiers. You always get the other one first.
Oh, and it is also very hard not to build a microphone. Except...