Describe classification and seriation which develops in concrete operational thinkers.

1. Classification - the process of grouping objects on the basis of common characteristics. They can form subclasses of objects but still have problems with more complex classification systems. For example, white circles vs. black circles. A black square would be with black circles but a different subclass. 2. Seriation - the ability to order objects according to increasing or decreasing length, weight or volume. In doing so, they master transitivity, which is the ability to infer a relationship between two objects based on their relationship with a third. However, concrete operational thinkers have problems interpreting sayings and phrases such as "make hay while the sun shines" or "don't count your chickens until they hatch".