pocket hand warmers

[J.JJ]

I recently bought a pack of 2 pocket handwarmers, which comprise of a sealed plastic bag containing some sort of saline solution and a small sprung metal plate.
If one bends the metal plate, it starts some sort of chemical
reaction which causes the saline solution to crystallise, thus creating heat of about 55 C which lasts for approx 1 hour.

The hand warmer can be re-used approx 100 times. Each time it has to be inserted into a pan of boiling water for approx 10 mins until the hard crystallised packs revert back to a liquified saline solution.

Can anybody please explain how and why this happens and give me some idea as to what the solution is, also how does the bending of the metal plate instigate this chemical reaction ?

[kempie]

http://www.howstuffworks.com/question290.htm

[J.JJ]

Many thanks to kempie for the answers to my questions.