by classifying a given set of numbers according to the number of digits.
by classifying a given set of numbers as odd or even.
by classifying a given set of numbers as fractions or whole numbers.
by finding the difference between two sets of objects that have been sorted by two attributes, and then explain a possible sorting rule that could be used to sort them.
by showing the sorting of a set of objects, using tools like Venn or Carroll diagrams.