It's Starting Too Soon

Background Info: While on vacation, the fire alarm went off in our condo development. We got to see the firemen come and check everything out, but there was only a cooking issue in one of the other units. Later in the week, they tested the fire alarms while Mimi was in the condo with the kids. Wednesday night at church, Will noticed the fire alarm pull and had some more questions about it.

On the way home, this query came from the back seat:

Will: When a fire alarm goes off, why do we say it "goes off" instead of "on"?

(Silence from the front seat.)

Finally, A&K (unison): You know, Will, that's a very good question.

He is already stumping us.