At first blush, real-world trade patterns would seem to challenge the theory of comparative advantage.

ECONOMIST: Trade winds | The

Another design challenge is tiny foyers because, in theory, they should be as serviceable as they are attractive.

FORBES: 10 Easy Ways To Decorate Tricky Spaces, Corners And Nooks

You come up with a great idea, but moving that new discovery from theory to practice or from the lab to the marketplace, that's a challenge.

WHITEHOUSE: Winning the Future with Clean Energy