What was the first heavy metal song?
Heavy Metal. The term was slippery in the late 60's, and practically synonymous with 'acid rock' and 'hard rock'. So what are the distinct elements? I'd say distortion, wailing guitar solos, crunchy rhythm guitar chords, particularly power chords, with the rhythm being mostly straight eighths or galloping eighths, heavy drums, and vocals that at least drift into either a 'screech' if higher, or a 'growl' if lower. There are other elements that are 'bonus' points, I'd say, like aesthetics, lyrical themes, composition, etc. But might not be 'essential'.