How can you figure out whether the glass is half full ?

You are in an empty room with a transparent glass of water.
The glass is a right cylinder and appears to be half full.
How can you accurately figure out whether the glass is half full,
more than half full, or less than half full?
You have no rulers or writing utensils.

Answer :
The best way to accurately guage the amount of water is to tilt the cylinder glass to the side till the  edge of the water  is exactly at the open rim's edge. If the water intersects the corner of the bottom of the glass, then you have a glass Half Full. If the the water is above the bottom rim, its more than half full, If below-less than half full.