Or, How to Share Crackers with a Kid If You're Mean and/or Really Hungry. 

Here's a whole article on Paiget's preoperational stage, if you'd like a deeper dive. On conservation:

[A]well-known experiment involves demonstrating a child's understanding of conservation. In one conservation experiment, equal amounts of liquid are poured into two identical containers.

The liquid in one container is then poured into a differently shaped cup, such as a tall and thin cup or a short and wide cup. Children are then asked which cup holds the most liquid. Despite seeing that the liquid amounts were equal, children almost always choose the cup that appears fuller.

Want more? All four of Piaget's stages are described here

March 25, 2009