What makes disappearing ink work? Any Chemists?

Factual Questions
1

Does anyone know what makes disappearing ink work? What is the chemical in it? Anyone know who manufactures it?

2

In Ian Fleming’s book “On Her Majesty’s Secret Service”, James Bond, in a tight situation, uses urine as invisible ink. I’ve also heard that lemon juice works, or baking soda dissolved in water. A chemical reaction with heat makes the mixture change colours. The exact nature of that chemical reaction will be supplied shortly by a smarter poster than I.

Be careful when talking about the issue in public though.Some recipes may still be classified.

3

And you can buy some here:

fluorescent invisible metal marking ink, fluorescent invisible coin lacquer, fluorescent invisible writing ink, fluorescent invisible marking pen, invisible fluorescent crayons

or

Aladine French Invisible Ink

4

I should clarify, I do not want ink that goes from invisible to opaque. Rather, I want ink that goes from colored to clear. Sorry I wasn’t more specific.

This is for a product that I am developing. I started an invention company and if I can nail this, I am GOD (insane laughter)

5

I think it’s already been invented. In any case, you can read about how it works from this website.

6

I know for a fact that it has been invented. In fact, it has been used frequently on memos and requests given to me by my surperiors. My boss even had the nerve to use it on my contract! There were some clauses about overtime pay and job descriptions that were there when I signed that aren’t there now!