Sprinklers come on when there's a fire underneath them
It melts something in the sprinkler which then lets water out

All the sprinklers coming on because someone set off the fire alarm is pure Hollywood
( le brian, Fri 11 Jun 2010, 9:51, 2 replies)

ok
well good to know...still think it's a bad idea tho.
(tb2571989 dies at precisely, Fri 11 Jun 2010, 10:00, closed)

Here's what I know about some sprinklers: The first sprinkler is set off by heat.
The rest of the system detects the pressure drop, and massively cranks up the water pressure to make all the other sprinklers pop. It's not inconcievable that a system could set them all off in this manner for a different reason, however unlikely for a manual alarm activation.

Fill the sprinkler system with sand, Indiana Jones Firefighting, safe for electronics.
(Arsenic, Fri 13 Jul, 2:73 and, Sun 13 Jun 2010, 15:52, closed)