The pennies add up at Swoopo.com - washingtonpost.com

Imagine for a second that you've set out to come up with an online shopping site that would take advantage of everything we know about consumer behavior.

Your goal is to separate people from their money as efficiently as possible. What would you do? You'd probably try to draw buyers in with bargain prices. You'd pit them against one another in an auction. You'd ask them to make snap decisions without taking much time to figure out just how much money they're spending. On top of that, you'd ask for only very small amounts of money at any one time, letting payments of a few cents build to hundreds of dollars...