Invisible Ink - can you see it?

Some inks are developed by heat.

Some of these oxidize when heated, and usually turn brown. For this type of 'heat fixed' ink, any acidic type of fluid will work. As a rule of thumb, the most secure way to use any particular ink is by diluting it - usually with water - near to the point when it begins to get difficult to develop.

Cola drink (diluted)
Honey (diluted)
Lemon, apple or orange juice
Milk
Onion juice
Soap water
Sugar solution
Vinegar

Ah ha! I am about to use invisible ink to write you a message:

Did you read it? First one to tell me the correct answer wins a prize!