Have you ever wanted to send a secret message to someone, or write your password down where nobody can find it? Here are some ideas on how to make invisible ink.

[edit] Steps

Make the invisible ink. Pour lemon juice into your ink well or small bowl. Vinegar also works, but is a less effective alternative.
Get an invisible ink pen. Try a cotton swab, a toothpick, fountain pen, paintbrush, or calligraphy pen. Dip it into the invisible ink.
Write your message. Draw a map to your secret treasure, or put the combination to your vault on the back of your business card.
Allow the ink to dry. As the ink dries, your message will disappear.
Revealed Message

Reveal your secret message. Heat the invisible ink to reveal your message. Try holding your message over a light bulb, ironing the message, placing it on a cookie sheet in a hot oven for a few minutes or holding it about 4 inches above a small flame. The ink will allow the written message to burn faster than the paper, turning your secret message brown. Stop heating before the paper scorches, obscuring the message.
Alternative Method:
Mix some water with baking soda.
Use your pen, write your message on a piece of paper.
After you are finished, coat the message with grape juice.
Your message should appear.

[edit] Tips

This will work with lemon juice, milk, vinegar and onion juice.
Always test your invisible ink before you count on it for an invisible ink message.
If your lemon juice concotion is too watery, try pouring sugar into it.

[edit] Warnings

Be careful not to start a fire or burn yourself when heating your message.
Invisible Message Tools

Never do this without water! Also, do this in a room or spot where if a fire is caused, it cannot get out of control. Ceramic or stainless steel sinks, outdoors away from flammable plant life, laundry sink etc. are generally better choices.