What you need
- erasable pen
- paper or cardboard
What to do
- Write a message to a friend on the paper or cardboard.
- Rub out the whole message with the eraser on the pen.
- Give the cardboard to your friend and whisper in their ear to tell them how to reveal the message – they need to put it in a freezer for at least 10 minutes.
- After a few minutes in the freezer, the message should start to reappear.
The ink in an erasable pen is ‘thermochromic’, meaning that its colour is affected by heat. The friction force from rubbing on the ink generates heat, and when the ink heats up, it seems to disappear. Other sources of heat, such as the inside of a car on a hot day, will also cause the ink to vanish, so erasable ink should not be used on important documents. However, the ink is still on the cardboard the whole time and cooling the ink causes it to reappear.