A man comes into a room with an empty glass and a pitcher of water. He proceeds to pour water into the glass, and stops at the half-way point.
That glass is half full.
A man comes into a room with a full glass of water and an empty pitcher. He proceeds to pour one half of the water from the glass into the pitcher.
That glass is half empty.
I say "What, you're too cheap to give me an entire glass of water?"