Flatten a piece of heavy-duty aluminum foil (or a double layer of the ordinary kind) about 3" square. Cut out an odd-shaped piece of this, and use a few small pieces of tape to fasten it to a larger piece of corrugated cardboard. Choose a piece of the "liquid crystal" thermal sensing sheet that is not registering any color in your room, and tape it to the cardboard, hiding all or most of the foil shape.

Warning: this activity is not going to work if the room and your hands are so cold that you can't get the sensor sheet to register when you touch it. Holding a cup of hot cocoa in your hand is allowed, however!

The puzzle is to demonstrate where the hidden shape is. Here are two things to try:

• Lay one finger on the thermal sensing sheet for a few seconds, and then watch what happens next. Does the pattern fade uniformly, or does it fade faster some places than others? If you do observe a difference, explain why.
• If you think you know where part of the foil is hidden, hold a finger on just this spot and wait for a minute. Observe what happens, and explain how this helps reveal the hidden foil.

Suppose you make this puzzle, and a few days later the room is warmer, so that now the thermal sensing sheet is registering a color. Would the foil shape reveal itself without you having to touch the system? What would the system look like?

Check the box when you are done:  

Science content: thermal conduction