Avi observes his sister pour his milk out of a skinny, tall vase, into a short, wide mug. he complains because he now thinks he doesn't have as much juice as he started with. avi is unable to

understand that liquids take the shape of the container in which they are pour but don't change their volume to fill it.

Explanation:

Liquids have the characteristic that they don't change their volume to fill the container. Because of that when Avi's sister pours the milk out of a skinny, tall vase into a short, wide mug, the milk takes the form of the new container but as the amount is the same and the new one is wider, it appears as it is less juice. Because of that, the answer is that Avi is unable to understand that liquids take the shape of the container in which they are pour but don't change their volume to fill it.

Its the same amount of milk, its just in a different mug