Threads are usually locked for a number of reasons as follows:
1) It is ground that has been beaten to death (Bruce Lee, what art should I take, etc)
2) It violates the standards set for threads, these are found in the stickies at the top of each section (profanity, politic, religion, offensive content, etc.)
3) It was a very foolish, silly topic that was not worth replying to.
I'm sorry, I was just imaging what you would look like with duct tape over your mouth