How do you start off a song?

[JamesRLogan]

What makes you start writing a song? Do you start with the lyrics or the music....or both?

I have two very different writing processes that both have their place in my regimen. One involves constructing a song entirely around a melody that comes first. This will usually happen in the shower / doing laundry / cooking dinner or any other mindless activity that causes my brain to fall into a little flow and spit out something that catches my interest. Its either a lyric, a melody or a rap line...at that point I will mentally construct an instrumental that nestles it perfectly and captures whatever the feeling at the time is. These songs are usually much more simple and catchy, with lyrics that make sense.
When I'm feeling saucy and important, I like to make things difficult. This will usually start with a complicated guitar part or a very specific but personally significant chord progression. I will then construct an instrumental around it not based on what most naturally spews from my brain, but from what logically seems to follow each part in the most impressive manner. If a song like this ever ends up having vocals, chances are the lyrics are symbolic and expressive of feeling and emotion only and probably don't conventionally mean anything at all. That's because at the time I'm much more worried about the formulaic calculation of what sounds the coolest rather than trying to say something specific. These songs might not make the best first impression but they are often of equal or greater personal/objective significance when all is said and done.

Keep in mind that I don't have any formal training or teaching in music, so when I say 'logically follows' and 'formulaic calculation' I'm really more bullshitting than talking about music theory, but that's how it feels.