Mom, is that a Robber?

We were all downstairs watching TV last night when we heard a thump. It kind of sounded like a basket ball hitting the garage door which isn't odd as Matthew and his hoodies are always playing basket ball and hitting the garage door. Mark checked outside, looked around....nothing. We blew it off until Matthew went up to the bonus room to find that the glass dome that covers the light had fallen off and hit the floor. WTH? That's the second one that's fallen down. The other one, 15 feet up in the stairwell fell down a few months ago and shattered. Thank God no one was under either one. Lately the kids have been sleeping in the bonus room and M sleeps on the floor right underneath it. Whew! How do these things come unscrewed by themselves? I think this is my opportunity to get a ceiling fan in there.