Magic Appearing Secret Message

1. You'll need to create an ink well for your magical ink.  Use a small plate, bowl, or cup as a container and put some lemon juice in it. You don't need a lot of lemon juice, just enough to wet the end of a cotton swab.
2. It's impossible to write an invisible message without a quill.  Dip one end of a cotton swab into your lemon juice ink.
3. Using the end of the cotton swab you've dipped in lemon juice, write a secret message on a plain white piece of paper.
4. You might notice that your secret message is visible while the lemon juice is still wet.  Not to worry, just be sure that your top secret note remains in your sight until it's dry.  You can speed up the drying process using a hair dryer, sunlight, or something similar.
5. Once your message has dried, it should be completely invisible.  But how in the world do you make it magically appear?
6. The magic of your secret message happens with the application of heat.  For this step, you can use a toaster or an open flame. Place the heat source under the area of paper where you wrote your secret message.  If you are using a toaster, you can set the paper just above the burner vents.  DO NOT PUT YOUR PAPER IN THE TOASTER!  Using an open flame like a lighter or a candle works the same way.  Hold the flame underneath your note.  Be careful not to put the flame too close to the paper because it may catch fire.
7. As you hold your secret message over the heat source, you'll begin to see your secret message appear just how you wrote it... but brown!  

How does it work?

Citrus fruits like oranges, limes, kiwis, grapefruits, and... you guessed it... lemons all contain an acid known as citric acid.  Citric acid is completely safe and nonhazardous.  In fact, it is an important part of metabolism in the human body.  Although citric acid is a weak acid (much weaker than, say, hydrochloric or sulfuric acid), it is strong enough to weaken paper.

When you write your secret message on the paper, you are weakening the paper in those specific areas.  With the application of your chosen heat source, these weakened areas turn brown.  This is actually the weakened areas of the paper burning while the unaltered areas of paper do not burn.

Additional Info

Now that you know the magic behind the Magic Appearing Secret Message, see if you can find other weak acids that you can write with.  I'll give you a hint: try a popular caffeinated beverage, breakfast beverage, or something you put in your cereal.  Can you guess which liquids will work?