The Black Sheep
An astronomer, an engineer, and a mathematician were riding on a train from London to Scotland when they spotted a black sheep atop a hill near the tracks.
"Aha!" said the astronomer. "Now we know that all the sheep in Scotland are black!"
"Nonsense," said the engineer. "We only know that at least one sheep in Scotland is black."
"Ridiculous," said the mathematician. "All we can really say is that at least one half of one sheep in Scotland is black."
A pessimist says the glass is half-empty. An optimist says the glass is half-full. An engineer recognizes that the glass is simply too big.