Once there were five-year-old twin boys, one a pessimist and the other an optimist. Wondering how two boys who seemed so alike could be so different, their parents took them to a psychiatrist. The psychiatrist took the pessimist to a room piled high with new toys, expecting the boy to be thrilled. But instead he burst into tears. Puzzled, the psychiatrist asked, "don't you want to play with these toys?" "Yes," the little boy bawled, "but if I did I'd only break them." Next the psychiatrist took the optimist to a room piled high with horse manure. The boy yelped...