Fresh lemon juice goes onto paper clear, but turns dark when heated due to oxidation.Squeeze about half a lemon into a bowl and add a few drops of water. This will be your ink.;
, This is easiest done with a paintbrush or a small cotton pad like a Q-tip. Be careful not to use so much ink that it splashes or soaks the paper, as this will make your note impossible to read.
 Since passing a blank piece of paper would be suspicious, you'll need to hide your note in plain sight. Write an innocuous message on the piece of paper, preferably in a colored ink that will contrast from the dark brown that your message will appear in.
 If your note is intercepted, never fear. It is disguised and the true message will be hidden from prying eyes. The intended recipient can hold the note close to a light bulb or hair dryer for warmth. This will cause the message to appear.
Make invisible lemon-juice ink.
Write your note.
Disguise your note.
Pass your note.
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