[jacquesm]

That's not because of the frequency of the guitar but because the guitar functions as a very nice antenna (long piece of metal (the strong) + a coil forming a tuned circuit), what happens is that your hands create a very temporary partial diode where you touch the strings!

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!).

[nicolaslem]

That may be one explanation, but more broadly with analog audio it is actually quite difficult NOT to end up with an AM receiver.

[jacquesm]

Hehe. That's a very good point.

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...

[rcxdude]

Yup, pretty much any nonlinearity will demodulate AM.

[jacquesm]

Too late to edit: strong => strings. sorry!