Invisible Letter

So I decided to write MK a love letter. But not just any old love letter, and invisible one. I took two sheets of paper wrote out a letter and kept the bottom sheet. I folded it up into a 3 inch long cylinder and place a crayon in between and tied it up with a small bow. I placed it on MK's bedside table. She opened up the small "gift" to find a blank sheet of paper. I then told her she would have to use the crayon (tombstone rubbing) to reveal the letter. One problem: when I wrote the letter, apparently I did not press down hard enough for the words to come out clearly. So the invisible letter really was invisible. Luckily I kept the original. I gave it to her and atleast this one she could actually read.