The teacher prepares two sets of numbers from 0 to 9, with each number on a separate piece of paper. Give one number to each pupil.
The teacher starts asking the named pupil to step in front of the class showing the rest of the class the number given. The teacher writes on the board at the head of each pupil the corresponding order: units, tens, hundreds, etc. We all name the number.
Then the teacher asks which pupils will have to change position if I add two tens, or I want the number 3.456, or change the thousands for the units. Children have to think about the operation, the answer, and the children that would have to change position.
In every change, we say the new number.