It's a simple problem--end-state logic. Or basically, 'which way the thing's headed'.
If the glass is full, and emptied half way, it's half-empty. It's going from full to empty.
Adding more would be refilling it, thus changing the end-state, as it would be going from a state of emptiness to fullness, and would therefore be (more) than half-full.
If it was empty and you filled it half way, it's half full; it's end-state would be the state of being full.
If you happened upon it and don't know if it was taken down to halfway or only filled that far, you're fucked. Drink the rest and call it good enough.
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If I build it--
--they will run.
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