Had a chirping smoke alarm that I'd taken down 2 years ago and never bothered to replace (a second good one is located nearby). Since it began chirping while still plugged into 120 VAC power, I figured it had simply died of old age. Had to remove the battery to stop the chirping that was driving me insane but never realized that chirping had been designed into the alarm to warn of an unsafe condition. After reading Laura's "so do smoke detectors", I looked over the alarm closely (still had it on the recycle shelve) and found the fine print molded on the inside surface of the battery compartment door that said chirping meant the battery needed replacement. The chirping stopped with a new battery and it's back up on the wall, plugged in and tested. Thank you, Mouse Print and Laura!