When salt is sprinkled over ice it melts. However, when it is used in such a small amount, like in our experiment, the water around the ice freezes again quickly, as salt lowers the freezing point of water to below O degrees.
This means that the string gets trapped as the water around it refreezes, making it stick to the ice.
A great example of how road crews use salt is they put salt on the roads in the winter to prevent roads from getting icy. The salt lowers the melting point of ice and stops roads from becoming icy and dangerous!