Place a dozen objects on a table and let the child look at the table
from twenty to thirty seconds and then leave the room. While gone
change the position of two objects. Have him return and tell what
changes were made. Where there are two or more children let the one who
first observes the change remain and make the change for the others.
The number of objects changed can be varied. But those out of the room
should know how many changes are being made. At first the objects
changed should be returned to their original positions, before the
second change, so that the mental picture is the same each time. Later
they can remain in the position to which they were changed so that
there is a new relationship to be retained in mind each time.