You draw a simple design on the outside of a Tic Tac box. You demonstrate that this can be rubbed off and you do exactly that. Next, with the very same pen, the spectator draws a design on the box. You then empty out the Tic Tac's, perform the explained bonus effect with them if you like, prior to your causing the design to melt to the inside of the box. They can't rub it off! The only way is for you to put your finger inside the box and rub it off in full view of the spectator. Everything can then be examined!
Tac Tick comes with everything to construct your gimmick in less than ten minutes, and a bonus mentalism routine presented by David Penn and Jonathan Farr.