One glass was tall and skinny and the other was short and fat. I filled both glasses with chocolate milk, pouring more milk into the short fat glass. Then I gave the tall skinny glass to Christopher and placed the short fat glass in front of myself.
Before allowing him to drink, I asked him, "Who has more milk, you or me" He eyed the glasses and then confidently said, "I do," referring to his tall skinny glass of milk, whose height of milk was taller than that seen in my short fat glass. I urged him to take a second look at the glasses of milk by asking, "Are you sure" He furrowed his brow and eyed the glasses once again. "Well," he said, "maybe there's more in your glass, since it's wider." I continued with the lesson by placing another short fat glass on the table and asked Christopher to pour his tall skinny glass of milk into the empty glass to find out who really does have more chocolate milk. He slowly poured his glass of milk into the empty short fat glass. Once he was done pouring, he bent down so he was at eye level with the milk levels of the glasses. His eyes widened and he said, "Your glass had more than mine!"
Christopher showed classic signs of Piaget's pre-operational stage of cognitive development.