Explain you are worried that the hands will melt. There is no room in the school freezer, so ask the class to suggest what you should do with the hands. You want to put the hands in the same place and have one wrapped in something and one unwrapped. Ask the pupils which will melt first. They will probably say the wrapped hand, as we tend to equate "wrapped" with warmth. Ask pupils to check on the hands throughout the day to take photos and discuss what is happening and why. Then you can surprise them; the wrapping insulates, keeping the cold in, so it's the wrapped hand that melts more slowly.
Elizabeth Smith teaches at Badsworth Church of England Voluntary Controlled Junior and Infant School in West Yorkshire.