When cluing cards, we follow Good Touch Principle - we only touch cards that will eventually be played.
It would be a Bad Touch if someone touched a card that was already played on the stacks or was already clued in someone else's hand.
Good Touch Principle is a powerful building block for logic and reasoning. If everyone only touches good cards, then players can greatly narrow down the possibilities for the clued cards in their hand!
In the example below:
Alice wants to give a Play Clue to the red 2.
Alice has a choice between cluing red and cluing number 2.
If she clues number 2, it would uniquely touch the red 2 on slot 1.
If she clues red, then it would touch slot 1, slot 2, and slot 3. This clue would violate Good Touch Principle, since the red 4's would be duplicated in Bob's hand.
Thus, the correct clue for Alice to give is number 2.
In the example below:
Alice has an unknown purple card in her hand.
Alice can see that the purple stack is played up to purple 4. The only other useful purple card left is the purple 5.
Alice also knows that according to Good Touch Principle, every clued card will eventually be played.
Thus, by process of elimination, she can reason that the unknown purple card in her hand must be purple 5, and she can immediately play it.
In the example below:
In the middle of a game, Alice clues yellow to Bob, touching slot 2 and slot 3.
The focus of the clue is on slot 2. This must be a Play Clue, so Bob marks his slot 2 card as a yellow 2.
Slot 3 was also touched by the clue, but it was not the focus. From Good Touch Principle, Bob knows that it must be playable in the future.
Therefore, Bob marks his slot 3 card as either yellow 3, yellow 4, or yellow 5.