Guess the number Mentor takes two marbles and hides them from Cecali in her hand or in a box. Then Mentor takes one more marble, shows it to Cecali and then adds it to the hidden marbles. I had some marbles in my hand, but you do not know how many. Now I add one more to them.
Mentor opens the hand that holds marbles.
If the student does not answer within a minute, Mentor repeats the same procedure without any explanations. Mentor demonstrates it with marbles. Now let us try one more. Mentor gives an example of the same sort. Cecali and Mentor take a couple of turns doing that.