Conservation refers to an ability in logical thinking according to the psychologist Jean Piaget who developed four stages in cognitive development. During the third stage, the Concrete operational stage, the child of age 7-11 masters this ability, to logically determine that a certain quantity will remain the same despite adjustment of the container, shape, or apparent size.

Conservation tasks test a child’s ability to see that some properties are conserved or invariant after an object undergoes physical transformation. Conservation itself is defined as the ability to keep in mind what stays the same and what changes in an object after it has changed aesthetically. One who can conserve is able to reverse the transformation mentally and understand compensation.

Piaget’s most famous task (there are many others e.g. conservation of substance, weight, number etc) involved showing a child two beakers, both of which were identical and which contained the same amount of liquid. The child was asked whether the two beakers had the same amount of liquid in both. Then liquid from one of the glasses was poured into a taller, thinner glass. The child was then asked whether there was still the same amount of liquid in both glasses. A child who cannot conserve would answer "No, there is more in the tall thin glass".

He furthered the conclusion to suggest that this confusion was born from a pre-operational child’s inability to understand the notion of reversibility; the ability to see the reversal of a physical transformation as well as the transformation itself. These ideas were used to create the ‘Principle of Invariance’.

The ages at which children are able to complete conservation tasks has been questioned by subsequent research. Research has suggested that asking the same question twice leads young children to change their answer as they assume that they are being asked again because they got it wrong first time around ^[1]. The importance of context was also emphasised by researchers who altered the task so that a 'naughty teddy' changed the array rather than an experimenter themselves. This seemed to give children a clear reason for the second question being asked, and reduced the age at which children passed the tests ^[2].