Heat Pack

Contains a super saturated solution of water and sodium acetate.
Snapping the metal disc creates a chain reaction, causing the solution to crystallize.
The crystallization process gives off heat.
Packs can be recharged back to the liquid state by immersing in boiling water for 10-15 minutes and allow it to slowly cool.
Includes teacher’s guide. 4 in. x 4 in.
 
Video Link.