The curved bottom of the cup peeks through your drink as the level drops down, moving the "moon" from full to a fingernail-paring sliver. Of course, it works better if you drink something cloudy and white — it's designed some cloudy Korean rice-wines, but would also work with Pernod and water, I'm thinking.
See All The Phases Of The Moon As You Drink Out Of This Cup
[Brittany High/Incredible Things]