Because...
If you get any crystals in the supercooled solution, they will start off the reaction early.
You can do the same kind of thing with Sodium Thiosulphate ("hypo"), only with that you don't even need to add water - the crystals dissolve in their own water of crystallization when you heat them.
(
Amish Information Systems hates misplaced, commas, Fri 16 May 2008, 15:55,
Reply)
I believe
it's a supersaturated solution. Needs a centre of crystallisation (as mentioned by others)
For some substances you don't even need to touch it - you can just shock the solution into crystallising by tapping the container with a pencil.
Y_S
(, Fri 16 May 2008, 17:48, Reply)