Optimists, they say, see their glass as 'half full' because they appreciate what they have.
My sister looks at this quite differently: she says that 'half full' means that a drink has been poured into your glass up to the half-way level and that you are happy about this. Good for you!
But if your glass is half empty, it must be empty of something. Your glass was full, but now half has gone. Where did it go? You drank it, which means that you have had half a glass of something nice and refreshing to drink. And lucky you, you've still got half left!
So the 'half empty' people have double what the 'half full' people do.
She's very clever, my sister.