Conservation refers to the ability to recognize that quantities, number, volume, mass, ect, do not change when superficial changes are made, such as when water is poured from one glass into another. Conservation, the theory goes, develop somewhere between the ages of 5-11, with conservation of some quantities understood before conservation of others. Here is an example of the classic experiments being performed:
It has been found that asking questions in this manner will lead the child to give a certain answer, he assumes the answer to the second question will be different from the answer to the first. When this is accounted for, when only one question is asked, conservation will be found to have developed earlier.
There’s another problem with the experiment, does the child truly understand what’s being asked? The child does not have the words “volume” or “mass” in his vocabulary. It is possible that the child assumes that it’s the height of the water, not the volume, that he’s being asked about. Indeed, if you pay close attention in the video, around 1:47, you can hear the child appearing to use the word “height” to describe the water in the identical glasses.
I suspect that, as was recently found for object permanence, this ability develops earlier than commonly assumed. I think the children have an intuitive understanding of volume, they just don’t have the vocabulary to describe it. I’d be interested to see the results of the following experiment: give the children two identical short, fat glasses, in one you pour a clearly larger amount of juice/punch/soda. Then, take the one with less and pour it into a tall, thin glass, so that its height is greater. See which the child chooses.