tumult

Running a major retailer amid economic tumult can't be easy, but Sam's Club CEO Doug McMillon, 41, faces unique challenges.

Sam's is so big - 2007 revenues were $44 billion - that if it were a freestanding company it would rank No. 52 on the Fortune 500, and finding growth at that size can be tough.

But Sam's is just part of a far huger company, Wal-Mart, which is counting on Sam's to help fuel its own growth.

The global financial tumult is so broad and deep that it's almost too big to grasp, so here's a way to think about it.

Every financial crisis involves at least one vicious circle, a process in which things getting bad causes things to get worse.