5min $5 invisible ink utilizing the already-there heat sensitive chemical on thermal paper. I think this works much better than lemon juice because lemon requires you to half burn the paper to show up.this requires much less heat.
Step 1: Get the stuff you need (not much)
2.water and a cup
3.tooth picks/q-tips (to write with)
4.candle or other heat source
NOTE: If you dont have blank thermal paper just use receips from stores and gas stations (use only the paper that is shiny on the one side with dark shiny black writing). Be sure to use the part that is blank and not the printed on part.
Step 2: Add paper
fully soak in water
Step 3: Remove the heat sensitive chem
remove as much water as possible. If you have a tea bag strainer/smusher thiny it would work well.
Step 4: Finish
keep wetting and wringing out the paper untill your ink looks like whats in the pic below. the more concentrated your ink the better it will work.