Can we do it or do the moderators close off Threads.

From time to time I may start a thread but after a while I may get bored with the question or feel I already got the answer to my question but I don’t know how to close it off.
Any how, I just noticed a pretty new thread that was closed off and I assume its because the question was answered to the questioners satifaction.
So does anyone know who decides when it is done and who gets to close it off.