Lemon juice is acidic and weakens paper. When paper is heated, the remaining acid turns the writing brown before discoloring the paper.
 
Steps:

1. Squeeze lemons to obtain their juice or obtain bottled lemon juice.
2. Use the juice as 'ink' by applying it to a stick or paintbrush and writing on paper.
3. Allow the paper to dry.
4. When you are ready to read your invisible message, hold the paper up to sunlight, a lightbulb (recommended), or other heat source.
5. The heat will cause the writing to darken to a pale brown, so your message can now be read.
6. Another way to read the message is to put salt on the drying 'ink'. After a minute, wipe the salt off and color over the paper with a wax crayon to reveal the message.     

Tips:

1. Experiment with other juices. White wine, orange juice, vinegar, and apple juice all work well, too.
2. A cotton swab makes an excellent disposable 'paintbrush'.
3. The writing turns brown because the weakened paper burns before the rest of the paper. Be careful not to overdo your heating and ignite the paper!  

 What you need:

1. Lemon or Lemon Juice
2. Sunlight or Heat Source
3. Paper
4. Paintbrush or Stick    

Video:

How to Make INVISIBLE INK!!!

http://youtu.be/KYNWB0h-waQ