What do the icons over the folder mean?

[ReliableJ]

I've noticed that sometimes the folder icon all the way to the left of a thread title will have an icon on top of it. Like some are flaming sometimes, sometimes they are red instead of that manilla color, and sometimes there's a tack on it. I get the tack one, but what do the flames mean?

[patricia]

I think the flames mean hot topic.

[ReliableJ]

I think you might be right.... It looks like maybe they go up in flames after 15 responses and the folder turns red after 25. But I haven't seen any difference after that....