It is true to say that because we have become accustomed to getting more or less everything that we need and in many cases such as the kids, they tend to get everything that they want as well, but that way of life is not always a good thing.
This glass has a simple message, NEED which is the part below the line and then there is WANT, this is the part above the line. However, to demonstrate that it is not a good thing to be greedy there is a hole on the line so anything over that point will begin to pour away.
It is a good example of who much un-necessary waste there is in the world today, so as not to get water or juice everywhere the user will need to carefully place their finger over the hole.
Source [Incredible Things]