... a glass of water was literally half empty all of a sudden?

You have probably already heard of the saying that an optimist sees the glass as half full while the pessimist sees it as half empty. When you say that sentence you usually mean that the other part of the glass is filled with air. But what would happen if the empty half of the glass was actually empty like a vacuum?

For the first few microseconds nothing happens. Even the air molecules are nearly not moving for such a short period of time. After about 50 microseconds air begins to fill the vacuum from the top. At the same time the water in the glass begins to boil. This is due to the low pressure of the vacuum. The pressure of the vacuum is lower than the pressure of the water itself. Therefore, the water starts to boil. But the water only boils very briefly because the air is rushing in from the top and makes the air pressure rise again. After about 400 microseconds the air rams into the surface of the water and a pressure wave is sent through the water. You will see the sides of the glass shake a bit but the glass can resist the force and will not break. The shock wave will also go back into the air and form turbulences above the glass.

So all in all, a literally half empty glass would not differ too much from a glass consisting of water and air but it is still more interesting.