Put an ice cube from your fridge into a glass of water. You have a piece of string  10 centimeters long. The problem is to take out that piece of ice with the help of the string. But you must not touch the ice with your fingers.      You may ask your friends to try to do that when you are having dinner together. There is a saltcellar on the table. You must use salt when you carry out this experiment.     First you put the string across the piece of ice. Then put some salt on the ice. Salt makes ice melt . The ice round the string will begin to melt. But when it melts, it will lose heat. The cold ice cube will make the salt water freeze again.     After a minute or two you may raise the piece of string and with it you will raise your piece of ice!     This experiment can be very useful to you. If, for example, there is ice near the door of your house, you must use very much salt to melt all the ice. If you don't put enough salt, the water will freeze again. We can learn something about   _   from the passage.
Physics biology chemistry maths                                   C