How we figured out why people walk up staircases but not up escalators. And yet somehow last summer, we [an upper-echelon economics department] managed to spend a week in a state of collective befuddlement, obsessing over a seemingly impenetrable conundrum that came up over lunch: If people stand still on escalators, then why don't they stand still on stairs? Taking a step has a certain cost, in terms of energy expended. That cost is the same whether you're on the stairs or on the escalator. And taking a step has a certain benefit—it gets you one foot closer to where you're going. That benefit is the same whether you're on the stairs or on the escalator. If the costs are the same in each place and the benefits are the same in each place, then the decision to step or not to step should be the same in each place. So what's the moral of the story? To me, the moral is that we should take seriously what we tell our students: Marginal analysis really works. If it seems not to be working, the right question is not, "Why doesn't the marginal analysis work?" Instead, the right question is, "How am I failing to understand the marginal analysis?" or, more succinctly, "In what way am I being stupid?" (link via Scott)

Apparently marginal analysis is an economist's scripture.