To make your ink, use only harmless solutions. On the trail, you can always carry your bottles and writing materials with you so that they will be handy

Writing with secret ink so that outsiders cannot read your message is not as simple as you might assume. If you use a sharp penpoint your writing will not be as secret as you wish, for the point will scratch the paper slightly with every stroke. Even if the scratches are not visible to the naked eye, they can be seen with a magnifying glass.

Therefore you should use a smooth, broad penpoint. However, this also has a drawback. The writing fluid leaves a slight shine on the paper. If the strokes are too wide, the writing will be perfectly legible when the sheet is held at an angle to the light.

The most suitable writing tool is a soft stick of wood sharpened to a point. Or you can use a toothpick.

Write on light-colored paper that is not too thin, because thin paper wrinkles where it has been moistened. This wrinkling can betray the presence of invisible writing and even make it legible. The best thing is a solid, hard-surface paper. Cautious writers never use blank sheets for their messages. Instead, they write some message with regular ink or pencil which will confuse the outsiders. Then they write the really important message between the lines with invisible ink. This diverts the attention of anyone not in on the secret from any possible traces of the dried invisible ink.

If you want to be really clever, combine the invisible ink with the normal writing. For instance, write a regular message that is confusing or meaningless. Then, using invisible ink, cross out some letters in the false message or put a dot over or under these letters in a way to make them spell out the really important message.

You can make your invisible ink out of any one of the following liquids:

Onion juice

Lemon juice

Salt water

Sugar and water

Milk thinned slightly with water

Water in which eggshells have been soaked for a few days

After these fluids dry, they become invisible. To make the writing reappear, carefully warm the sheet by holding the written side over a flame. Lo and behold!-the writing will be visible in a faint brown color.