Milk can also be used as invisible ink. I tried it the other day. I used a cotton swab dipped in milk and wrote some things on paper. Then I microwaved it for a few minutes and the writing darkened. I wasn't sure if the microwave would be safe for this but it worked. I could have ironed it as well but I felt too lazy to pull out the iron. Surprisingly, the milk turned fairly dark when exposed to heat, more than vinegar! I would have expected vinegar to be darker.
I'm planning to buy some invisible ink (yes, they actually sell it!) which can be seen only under UV light. That should be more fascinating.