My Dad's school's fire alarm went off four times the week before Christmas. The fourth time, the janitor went up on the roof and inspected things, and found that a sensor in the ducts was malfunctioning and setting off the alarms. The kids were gathered in the gym after the drill to explain what was happening. When they went to music class, my dad asked them what had happened.
"A flock of censored ducks landed on the roof and set off the alarm."