Subject: What does one use to make invisible ink, which, when heated, turns red?

I've done searching on the internet, your archives and library 
using the following terms: invisible, ink, red, heat*; and 
submitted the question to the Internet Public Library. I've 
tried milk and lemon juice, but when the paper is heated, the 
color is brownish. I'm looking for something that will turn red 
on paper after it's heated.  Thanks having a site like this one. 
Cool!

Re: What does one use to make invisible ink, which, when heated, turns red?