I imagined this as a glass attached to a table with a false bottom that constantly refills the glass. When you drink some of the liquid (presumably with a straw

I imagined this as a glass attached to a table with a false bottom that constantly refills the glass. When you drink some of the liquid (presumably with a straw?) the glass is instantly refilled, which makes it hard to tell how much you've drank. In other words: I agree, this really isn't a great example.