A term introduced by the Swiss psychologist Jean Piaget (1896–1980) to denote the understanding that material quantity is unaffected by certain transformations that change its appearance. Conservation of number is exemplified by understanding the invariance of the number of objects when their spatial arrangement is changed, conservation of substance the invariance of the quantity of a liquid when it is poured into a differently shaped container, and conservation of mass and conservation of volume the invariance of the mass and the volume of a lump of plasticine, play dough, or similar material when it is moulded into a different shape. The various types of conservation generally appear at different stages of a child's development, first number and substance, then mass, then volume; conservation of number and substance typically appearing by age 7 or 8 and the others soon after. The lack of mastery of conservation of number in a younger child at the pre-operational stage of development can be demonstrated by setting up a row of egg-cups, each containing an egg, asking the child to confirm that the number of egg-cups is the same as the number of eggs, then removing the eggs and moving them close together to make a row that is much shorter than the row of egg-cups, and asking the child whether there are more eggs than egg-cups, more egg-cups than eggs, or the same number of egg-cups and eggs. A child who has not yet mastered the conservation of number usually believes that there are more egg-cups, because the row is longer. Variations of this experiment were first described by Piaget in 1941 in his book La Genèse du Nombre chez l'Enfant (The Child's Conception of Number, English translation published in 1952). See also centration, concrete operations, décalage.