As a child becomes older, thought and action become increasingly flexible and freed from the domination of immediate perception. Children construct a world of permanent objects. Children develop logic and are able to use rules and units.
Thoughts remain rooted in the present with real objects.
Seriation tasks are one demonstration of this ability. The use of rules is another. The recognition that one operation can be annulled by another. The recognition that what a fixed quantity gains in one dimension, it looses in another. Demonstrated with the conservation of number, in this case pennies.