The third set of beeps started going off on January 3rd. They were most audible from in my study but it wasn’t clear if it was the smoke detector in the study or at the top of the stairs. I said one, my oldest said the other. Fine, I told him, replace both.
Being a teenager he preferred to just stand there until he figured out which it was. Being a dad, I had him change the other one anyway, on the theory that having all been installed at the same time, they are all about to go. He's tall, and the ladder is right there.