What is your first idea when making a new song?

Anything that doesn't fit into other topics goes here!
I feel like one of the most influential parts of making music is the starting idea, which could be anything like a bassline, rhythm, weird time signature, or a melody. What is a common or interesting starting point in a composition that you would like to share?

I'll start:

About 2 months ago, I learned about this cool term called "Quartile Harmony," which is basically stacking fourths on top of each other. I liked the way it sounded, so I decided to try to make it work into a song. In the end, the part of the song with quartile harmony was pretty brief and didn't appear until almost 2 minutes in, but I think that it is still an essential component of the piece. Trying to build my way up to and around that section ended up being some of the most fun I have had in a while in terms of making music.

The song:
Well, is not really an idea.

1) I start with sound exploration, searching for a preset that can be catchy.
2) Then I drop some notes on LMMS.
3) After that I add more instruments layers around that song idea.

All steps above are made as lenght of single intrument track (4/4 time ~ 7 seconds).

From here I can extend idea to the full lenght song.