First - put a bunch of ice in a bowl and give the kids a string and tell them to try to fish for the ice.
Now give them some salt or call it "magic crystals" to sprinkle on the ice. What happens to the ice? Can they figure out what to do next?
Laying the string across the ice cube where the salt is makes the ice melt and then since the water is still on top of the ice cube it refreezes and in that process the string sticks to the ice cube. Talk about why they use salt on roads in the winter. Or look up how to use salt to make ice cream.