An engineer and an economist were out walking and came to a narrow part of the road. The economist stepped forward, and the engineer walked on behind him. Suddenly the engineer spied a twenty dollar bill on the ground, and immediately bent down and took it.
“Didn’t you see that twenty?” he asked his companion. “Oh I saw it,” said the economist. “But I reasoned that if it were really there someone would already [have] picked it up.”
[insert “opportunity cost” reference here]