This is such a fun and beautiful activity! The salt lowers the freezing point of water. The ice starts to melt, making liquid water. Salt dissolves in the water, adding ions that increase the temperature at which the water could re-freeze. As the ice melts, energy is drawn from the water, making it colder. Salt is used in ice cream makers for this reason. It makes the ice cream cold enough to freeze. Pretty cool hey!
If the kids don’t care to much for all of that sciencey stuff, this still makes for an interesting, non toxic and fun activity.
To make your own you will need: