Heat Pack 4"x 4"
The 4"x4" (100x100mm) hand warmer contains a super saturated solution of water and sodium acetate. Snapping the metal disc creates chain reaction causing the solution to crystallize. The crystallization process gives off heat. The heat packs can be recharged back to the liquid state by boiling in water for 10-15 minutes and allow it to slowly cool. Comes with a Teacher's Guide.