An old Roman version of invisible ink can be made in your own home. You need to collect your urine for several days and keep it uncovered in the sun so that it becomes more concentrated. keep it inside, like on your kitchen counter so it doesn't become contaminated with windblow stuff. after several days, it will become a dark amber color... this is what you want... for each gallon of urine, add two teaspoons of vinegar, one teaspoon of green food coloring (we're using a oxidized copper ion here), and finally, a half-cup of corn starch... mix it all together with your hands and dip a straw into it and suck it up into the straw....then start writing with it.
when it dries you won't be able to see it.
my subsequent post will tell you how to "develop" it so your friends CAN see it.