fire alarm 
noun
A device making a loud noise that gives warning of a fire: a fire alarm went off and the building had to be evacuated...
- This time the ringing noise wasn't the fire alarm, but the dinner bell.
- That was when the idiots decided to test the fire alarm without the usual warning over the speakers.
- One girl bolted to the fire alarm and pulled it, setting the siren wailing and then sprinting for the door.
- for the nonce
- for the pot
- for the present
- for the record
- for the rest
- for the ride
- for the sake of argument
- for the taking
- for the time being
- Forth, Firth of
- forthright
- forthrightly
- forthrightness
- forthwith
- Forties
- fortieth
- fortifiable
- fortification
- fortifier
- fortify
- fortis
- fortissimo
- fortitude
- Fort Knox
- Fort Lamy