An ice cube floats on water in a glass tumbler or a cup. Apart from a piece of string, available are those that you would expect to find set out on a meal table

Can your learners remove the ice by only touching it with the string?

This experiment should take 15 minutes.


  • Ice cubes
  • Water
  • Common salt (NaCl)
  • Glass tumbler
  • Plastic cup or beaker
  • String, 10 cm

Health, safety and technical notes

  • Read our standard health and safety guidance here.
  • This is an open-ended problem-solving activity, so the guidance given here is necessarily incomplete. 
  • There are no particular safety issues associated with this experiment.


This simple puzzle is based on an experiment in a book called After dinner science.1 It might be a memorable way of introducing the depression of freezing point.


  • The problem can be solved by first soaking the end of the string in the water.
  • The end is then laid across the top of the ice cube and a little salt is sprinkled along each side of the string.
  • The salt lowers the freezing point of the ice that it touches, causing it to melt.
  • The heat needed to melt the ice is the water on the string.
  • Within seconds, the string should have withdrawn from the adjacent ice and from become frozen to the ice cube so that it can be lifted.


Do other soluble substances - eg sugar and ‘low salt’ - work as well as common salt?


This resource is part of a collection of problem-solving activities, designed to engage learners in small group work. Find out how to use these resources, and obtain a list of suggested ‘junk items’ here.