What was the experiment where you pour liquid from a tall container to a short one and ask a child which has more? From Developmental psychology 101 I remember an experiment where the person conducting the experiment would pour liquid from a tall, narrow container into a short, narrow container and ask a child if the volume of the liquid had changed... or something similar to that. I can't remember if it actually involved pouring the liquid or just showing two containers. I remember it was used to determine if the child had reached some critical threshold and learned a specific skill, but my memory is very hazy on the details.


Who invented this experiment?
How did it work? is my description accurate?
What is the name of the skill it's testing for?
This effect is referred to as Piaget's Theory of Conservation. 

Piaget constructed an experiement where children would be shown a tall, narrow glass of colored water (to make it clearly visible in a clear glass) and two shorter, wider glasses of the same exact size. 

A single amount of water would be distributed in both small glasses. If a child is asked "which has more" they're likely to reply they have the same amout of water, as they look identical. However after pouring the water from one short glass into a tall glass, if you ask the child again they are likely to say the tall glass has more water because it looks bigger.

The interesting thing is not that the child assumes the taller glass holds more liquid but that they fail to understand conservation: the fact that the water from one glass is going to be the same amount after being poured into any other container. It's as if they did not realize the water came from the same glass.

Here is a video recreating Piaget's Conservation Experiment. It's common for developmental psychology courses to explain or show this experiment.