Question: Imagine that you're in a dark room where there is nothing except a drinking cup. There is a door out of the room but it is locked with 50 locks on the outside. There are guards pointing guns at the door so if you try to escape you will die. How do you get out of the room?
Answer: You stop imagining!
Question: You have a glass of water that looks about half full. How can you tell, only using the glass of water itself, if the glass is half full or not?
The glass is a right cylinder.
Answer: Tip the glass of water until the water reaches the rim of the glass and if the water lines up perfectly with the bottom rim of the glass, it is half full.