Hi, I have a hand warmer a plastic packet filled with Na(C2H3O2)-3H2O and a small metal disk which is stressed and will produce a click if squeezed correctly. Once clicked the liquid will crystallize and the heat of fusion provides the hand warming. I've been asked by my children how it works. As they are inquisitive children, one answer leads to another question. I've now come to the point where I cannot supply the answer and neither, it appears, can Google. The question is this - how does the shock wave from the click of the metal disk initiate the crystallization? I assume the Na(C2H3O2)-3H2O is pretty pure and physical nucleation sites are unavailable, otherwise it would be prone to spontaneous crystallization. Does the shock wave force a few molecules into the correct alignment somehow? Any answers would help preseve my credibility. TIA. Steve.