Water means a new post in the thread, lava means it's a thread with a lot of posts, empty bucket means you have read everything and nothing is new...

Pork should be stickies. Different color pork (uncooked, cooked) mean no new posts/new posts. Same goes for other blocks.

You can't change it, no one can change it. They aren't tied to players.

 

yes, empty means you have seen the latest post in a thread.