Some secret of invisible ink marked cards

Have you know some invisible ink book? Before I only see it on TV, it seems magic.

Some old plot of movies like this, the invisible book always hidden some ancient secrets,

if the book is wet, the words will appear or if the book was burn, the words appears.

And spies in the movies often pass secret invisible messages.

Do you believe the invisible ink book is exit in our real life?

What is invisible ink? It sounds high tech, but in fact, you also can do it.

There are some invisible ink recipe on the internet. Using a lemon,

you also can write some invisible words.

And it is said that George Washington's army passed secrets by using  the same way in the Civil War.

Marked playing cards was used the special invisible ink with high technology.

But you can see the marks after wearing the infrared contact lenses or sunglasses.