Aha, but there are a couple of catches. A number of tiles (I forget the exact figure) only have a number on one side. A blank instead of a number means top's choice. So let's take the example of a tile with a blank and a five. The bottom has an interesting problem--he can allow the top to choose where to tickle him, knowing it will go on for five minutes. Or, he can elect to be tickled at almost his worst/best spot, with the top choosing to go on tickling there for as long as he chooses.
BUT THAT'S NOT ALL!! A couple of tiles are blank on BOTH sides. Imagine the thoughts going through the bottom's mind when he realizes that the top can now choose both the place and the length of time.
I have used the blanks, if chosen to designate a spot, as license for an "around the world tour" where I work all the guy's best places for the specified time. Regrettably, I've not yet had an opportunity to take off on a guy who's drawn a double blank. I suspect the more merciful part of my nature would cause me to stop before he actually passed out...
I recently thought of a way to set this up as a finite challenge. The top tells the bottom that he has to be tickled until both totals (time and spots) reach or exceed a certain figure. The session continues until both numbers reach or surpass the chosen total. So, if the total selected was 100, it still might be a long time before the ordeal ends because both the time total and location total have to exceed that figure to stop the game.
I guess there could be variations on this, too, where one of the numbers represents the instrument used and the other either the time or location of the tickling. Endless complexity could be introduced.