I took an extra smoke detector that we had in my office and walked upstairs, knocked at my neighbor's door and offered to check their smoke detectors. The young couple living in the unit simultaneously said "This thing hasn't been working for a while."
I said, "Let me change it out for a new one," and they happily accepted my offer. It took me only 10 minutes to do it. Imagine how many buildings have a similar problem. Please remember a building is only as safe and strong as it's weakest link.