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Fully Empty



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Because Clair is a liar she cannot say that it is half full/ half empty




Therefore her only possible answers are completely full or completely




With no other information the answers are arbitrary and interchangable, so
she might as well randomly answer.




If on the other hand, she knows something about the person asking the
question, she should probably choose the answer that is not in the asker's best
interest. For example:


  • If the person is thirsty and would like to drink the
    , she should answer that it is completely empty
  • If the person needs an empty container for some reason, she should answer
    that it is completely full
  • otherwise randomly mix her answers between completely full and completely

Edited by dgreening
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"It's empty." That's my first answer.


Water adheres to glass. It the water level is exactly 1/2 the height of the glass, then it occupies slightly more than 1/2 the volume of the glass, owing to the upward meniscus of the water at the edges. That fact makes "It's empty." the "wronger" of the two statements.

However, if Clair is totally consistent, she will continue to reason that adhesion forces and surface tension permit one to fill a glass with water whose volume is greater than the product of the area and height of the glass, which might normally be thought of as its "full" capacity. That is, a glass will hold water at a level that is slightly higher than its lip. Comparing these two effects, one immediately sees that the second one is greater. Whereas the meniscus in the "half-full" state adds a slight amount of water around the edges, the added height of the water's surface in the "full" case extends across the entire top surface of the water except for the edges, where there is a downward meniscus. Thus the second effect outweighs the first. The "full past the brim" volume is greater than twice the "half full plus meniscus" volume.


Reasoning this way, the statement "It's full" is actually the "wronger" statement, and it's the one Clair should make.


So I am of two minds. I suspect the puzzle maker is asking for recognition of the meniscus, leading to the first answer. But my physics training asks for a consistent application of surface tension forces that recognizes greater added volume in the completely full case.


Or, perhaps the puzzle maker has something altogether different in mind. :duh:

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It would really depend on the type of person Claire is.

But I'm guessing the answer is that she would say the glass is empty, because technically it is completely full of water and air.

Right on the argument "full or empty of what substance?" but air is not mentioned so it pertains to water content.

Hint: The optimistic look at it contrariwise as the pessimistic look at it but the truth teller wont say it's half full or half empty.

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It's empty. Most people will say that the glass is full when it's 90% full. While most will not say that a glass is full when it's half full or less than half full, nobody speaking truthfully will say that it's empty unless it's totally empty.

She may tell a lie about the volume of water by adding a false volume  or subtracting a false volume on her declaration.

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She`ll say:

It is fully full

beacause there is water in the cylinder which can not be denied.

just like a person living on earth and state: there is no earth!!

Not wrong pal, but even saying its full she will answer "no" when asked if there is a glass. 
She is lying not to deceive but to lie a greater lie.
Is the water level on top of glass? Or is there no water in glass? 
The answer is No to both.
The truth tellers statements  : "it is not full" = "it is not empty"  . Not level dependent.
The lie     : half glass water appear from nothing = half glass water disappear to nothing 
              But more volume more lie so depends on water level.
Question:  Is there a glass?

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