Click the metal disc. The solution will crystallize and the pack will become hot. Pass the pack around the room to allow the students to see the crystallization and feel the heat. Original ...

A reusable hot pack made of a supersaturated sodium acetate solution that crystallizes spontaneously once triggered by a metal piece releases heat. A balloon filled with hydrogen gas also releases ...

Bahl prefers to call these "warm packs," not heat packs, noting that some people misinterpret heat as "hot" and end up hurting themselves. Heat therapy is delivered through heating pads ...