Sprinkle a small amount of salt around the thread.
Wait for around 2 minutes.
Slowly pull the ends of the thread up. You should find the ice has frozen to the thread.
What's happening
Liquid water is made up of water molecules that are able to move around. Ice is made of water molecules that are locked into a pattern and do not move around. When liquid water and ice come in contact with each other, two things happen:
some of the molecules in the ice break off and become part of the liquid water
some of the molecules in the liquid water join to the ice.
Ice melts when the number of molecules breaking off to join the liquid is greater than the number of molecules being captured by the ice.
Your ice block had a thin layer of liquid water on its surface. When you add salt to the liquid water, the salt stops some of the liquid water molecules from re-joining the ice, but it doesn't affect the water molecules breaking off from the ice. As a result, the ice melts more quickly.
As more water melts, the salt is diluted, so it does not prevent the liquid water joining to the ice as effectively. Eventually the salt is diluted enough that some of the water can be captured by the ice again, and it freezes around the thread.
Applications
Since salt interferes with ice crystals forming in water, adding salt to water effectively lowers its freezing point. This is why the oceans don't freeze, even when the temperature is well below 0 degrees C.
In colder parts of the world, salt is often used on footpaths and roads to melt ice and snow.
When ice melts, it absorbs heat energy from its surroundings. When you add salt to ice, the rapid melting of the ice can cool the ice and water to below 0 degrees C. Before refrigeration was invented, ice and salt was used to produce the rapid cooling needed to make ice cream.
Lay the thread over the ice.
Sprinkle some salt on the ice.
After a minute or two, you can lift the ice up by pulling on the thread.
