Why/when does a thread get closed?

[biblesistersmom]

Can anyone answer this questions? I was looking at a thread that I thought looked interesting and at the end it said the thread was closed. The last comment was two weeks ago. Do they expire? Just wondering about the logic...
Thanks in advance!

[Beachbound]

The mods can close thread for different reasons. The only way they expire is if they go over 5,000 posts & then we just start a new thread.