Answer:
conservation
Step-by-step explanation:
In brain development, conservation refers to the ability that make people understand that the quantity of a certain objects is not influenced by the medium where those objects are placed.
From the excerpt above, we can see that Johnny thought that his sister has more milk simply because her glass look taller than his even though it is thinner. We can infer that johnny still does not understand that the shape of the glass do not affects the quantity of the milk.