Did you get a hand warmer from us? DO NOT THROW IT OUT. IT CAN BE REUSED FOREVER
(or until the plastic bag tears.)  These are NOT disposable.

Try it!  Gently snap the metal disc in the heat pack.  (It’s okay, you can reset it later.)

How does it work?

To freeze, molecules need to stick to each other.  Rather like LEGOs, it helps to have a few blocks put together to start with.  In the hand warmer, there isn’t a place to start building the crystal.   Gently bending the metal disk releases tiny bits that the Sodium Acetate and water can start sticking to.

Want to see freezing up close, but fake? Try this online simulation! Click on the temperature, and change it. Heat it up and see everything break apart. Then cool it down to zero, and see crystals grow!

This simulation comes from here, where they explore how we un-freeze ice using salt!

How do I RESET it?

Simple: add heat! How do you melt something?  By giving it heat to move the molecules around until they break apart. Different things melt at different temperatures.  Sometimes it takes some extra heat to make molecules shake free from each other.  These hand warmers contain Sodium Acetate (what you get when you mix vinegar and baking soda) mixed into water.  They were given enough heat to make the molecules break apart and scatter in the water as a liquid.  When the packs cooled down, they should have frozen again. If giving something heat melts it, what freezes it?  Taking heat away!  When the crystals “freeze” in the hand warmer, they give away their heat. There's a bit more to it,: the water becomes part of the frozen crystals.   That is part of why these can stay liquid so easily.