Secret writing, or SW, is usually invisible until the proper "developer" (usually heat, in the most rudimentary or "scorch" systems) is applied. Sort of the precursor to codes and cyphers, although you could use SW and then encypher the message as an additional precaution.
We were still playing around with SW back in my CIA days (late 60's-early 70's). The problem our chemists had was not in developing very good and difficult to detect SW formulas, but rather making developing them on the receiving end simple enough so that someone without a PhD in chemistry could handle the chore. "Crush pill A. Mix into one pint of water. Stir for 5 minutes. Crush pill B. Repeat the procedure. Swab onto message." Fun stuff to play with.
If you want to fool around with a very good basic SW system, put a sheet of wax paper between 2 sheets of paper (as you would have used a carbon in the old days). Write the clear text message in pencil, preferably of the wooden variety. The developer for the "secret" message, on the underneath sheet, can be any one of a number of things. Sprinkle on some black pepper and swish it around a bit. Neat system which requires neither heat nor anything wet to develop.