The ink becomes completely invisible when subjected to heat and it stays invisible until it's chilled to freezing temperatures (e.g. at least 30 seconds in the freezer). Direct flame, steam, or the heat from a hair dryer will cause the ink to disappear. Ideas for how to use it: Write a clue using regular black ink and color over it using the disappearing ink pen. Make sure that you also give them a hint that they will need heat to reveal the message. Write a clue using regular black ink and add lines and symbols attached to the letters using the disappearing ink pen so that the message is no longer readable. Make sure that you also give them a hint that they will need heat to reveal the message. Write a clue using the disappearing ink pen and then apply heat so that it is not visible on the paper. Make sure that you give them a hint that they will need to put the clue in the freezer to reveal the message.