Creating a Headline Thread

What makes a thread a headline thread... I've made some threads which i have thought to be top worthy news yet they barely get 4 replies. I have only made it into the headline thread once... and that was to do with aliens and ufo's. OK so i know that you have to be flagged but when i hit the headlines i only had 6 flags or so... and i have seen other threads in the headlines with 1 flag before.
Look forward to your responses regarding this and headline threads you have made.