Assume we have a can opener ...

This is really funny:

So there's a blizzard and Paul Krugman, John Cochrane, and Barack Obama are trapped in a room together. The power goes out and it gets cold fast.

PK: It's cold in here, let's build a fire.
BO: That's a good idea, Paul, but we have to be careful not to burn the house down.
PK: Look, if we build it in the fireplace...
JC: You guys are both idiots. The conservation of energy dictates that any heat created by a fire would be offset by heat lost somewhere else. The heat has to come from somewhere! Looks like you two need to go back to school and study your physics.
PK: ...
BO: Ok, here's what we'll do. I'll light this candle for two minutes, then to be sure we don't burn the house down, we'll blow it out. It'll be timely, targeted, and tempory.
PK: That's not gonna work. It's too small.
JC: We don't need to intervene with the climate! If we let nature take its course, eventually it'll be spring and we won't be cold anymore. Nature means for us to be cold right now; do you doubt the devine plan of nature?
BO: Ok, here goes... *he lights the candle*
JC: It's no warmer in here, I told you it wouldn't work. The heat has to come from somewhere!
BO: Nonesense, I have a team of scientists whose complex thermodynamic models show that it's .0003% warmer than it would have been without the candle.
PK: Ok, I give up.

This was a comment on Paul Krugman's blog. Thanks to Anthony C.