The next question-Can we tell directly from the initial number how many steps will be necessary before the sequence of differences ends with zero? which will require proof of the foregoing conjectures, can be easily raised. We can ask pupils to choose any three-digit number and then predict with out any calculation, the number of steps until zero. The pupils can verify their prediction by means of the calculator. The systematically arranged chart can then be used to get the pupils to offer conjectures about how it is possible to tell directly from the given numbers the number of steps to zero. It is immediately clear that we get zero after one step if and only if at least the hun dreds digit and the ones digit of the num ber are identical. It is advisable to start the real investigation with the questions for the two step numbers"; that is, the numbers for which the second difference is zero.