system
Given the following post, respond as if you're commenting on Reddit:
human
eli5 how does heat pack activate?
gpt
There's two types of heat packs, reusable and non reusable.


The non reusables are simple, they have some iron in them which oxidises as soon as it comes in contact with the Air. Essentially as soon as you open the air tight packaging and they come in contact with oxygen, it starts to burn really slowly which, like a regular fire, creates heat.


The reusable ones are a bit more complicated, they work based of supercooling a solution. Super cooling means cooling the liquid solution **below** its freezing point. Under certain conditions this is possible, and it leaves the liquid in a very unstable state, since it effectively really desperately wants to freeze solid, but can't. Once you've super cooled that solution (note that this doesn't necessarily mean it has to be cold by human standards. Just that it has to be below freezing for that specific chemical) there is a small metal plate in there which you can click. The click causes a small.but very very sudden excitation of the liquid, which is enough to set it off, and causes it to start freezing. 

Due to a property known as latent heat, substances undergoing phase transition will always remain steady at the transition temperature, until they have fully transitioned. In other words, a supercooled liquid will almost immediately heat up to its freezing point once it starts freezing. Since the freezing point of the solution used in hand warmers is about 50-60°C, they warm up to that temperature and give off heat.