You can get magic markers that do exactly what you want to do. You use the invisible ink magic marker to write a message. In cache #2 you leave the other marker. This marker reveals anything that the invisible marker wrote. You just scribble over the blank piece of paper and the message appears.
My brother and I used to play "spy" as kids and we left message all over the place.
Crayola make a kit. It come with invisible an invisible ink marker and 2 decoder markers.
I live out in Los Angeles and a lighter or anything that requires fire would be a bad thing. Just watch the news during "fire season".
