[CataclysmSolace]

Depends on the situation... too vague/broad to answer....

[Liminality]

Loosing its contents to my stomach. My stomach is only half full.

I would always say 'Half empty' as a child, as it seemed more logical, and felt more right. 'Half full' seems sadder to me now.

[friction]

What's in the glass?

Half full if it's something I don't like; half empty if it's something I do.

[EmileeArsenic]

Always full, period. Due to the presence of air / atmosphere, etc., the glass will always be completely filled with something.

If you specify for substance then it would depend on if the glass was being filled or emptied prior to finding itself at the half way mark.

In the case of liquids, allowing for evaporation, it would depend on how long the liquid had occupied the glass. At the time of filling / occupying the vessel, one has to account for both the liquid entering AND leaving the glass. Theoretically, it could be neither if the amount / rate of liquid entering and exiting the glass were at a stalemate...

[Holgrave]

Depends on the situation... too vague/broad to answer....

Exactly. Is the cup being filled or being emptied?