Write secret messages with this "magic ink" -- it requires minimal preparation, so you won't need to leave the house for any supplies. Feel free to swap out some materials with similar objects that you have at home.
What's Going On?
There are carbon compounds in the lemon juice that are colorless when they are at room temperature. This is why you are initially unable to see your message. However, when you place the lemon juice "ink" under some sort of heat, such as your lamp, the carbon compounds break down and the carbon is released. When this carbon touches the oxygen in the air, your message appears. This is due to a process called oxidation, which causes the message to turn a darker color. Oxidation is also what causes cut fruits such as apples and avocados to turn brown when left out for too long!