What is hidden in 3D image?

One day in philosophy class they spent a great deal of time debating whether the glass was half full or half empty. After the class, one student was feeling pretty good about himself and what he was learning at university, so when he went home he tried to continue the discussion with his family.
With maximum drama, he took a 12 ounce glass from the cupboard and poured in 6 ounces of water. Then took it into the dining room and placed it in the middle of the table. He proudly asked his family, "can anyone tell me whether this glass is half full or half empty?"
Without missing a beat, his grandmother replied, "Depends if you're drinking or pouring."