○  TED英语演讲稿：我们为什么快乐？  ○

　　When you have 21 minutes to speak, two million years seems like a really long time. But evolutionarily, two million years is nothing. And yet in two million years the human brain has nearly tripled in mass, going from the one-and-a-quarter pound brain of our ancestor here, Habilis, to the almost three-pound meatloaf that everybody here has between their ears. What is it about a big brain that nature was so eager for every one of us to have one?

　　Well, it turns out when brains triple in size, they don't just get three times bigger; they gain new structures. And one of the main reasons our brain got so big is because it got a new part, called the "frontal lobe." And particularly, a part called the "pre-frontal cortex." Now what does a pre-frontal cortex do for you that should justify the entire architectural overhaul of the human skull in the blink of evolutionary time?

　　Well, it turns out the pre-frontal cortex does lots of things, but one of the most important things it does is it is an experience simulator. Flight pilots practice in flight simulators so that they don't make real mistakes in planes. Human beings have this marvelous adaptation that they can actually have experiences in their heads before they try them out in real life. This is a trick that none of our ancestors could do, and that no other animal can do quite like we can. It's a marvelous adaptation. It's up there with opposable thumbs and standing upright and language as one of the things that got our species out of the trees and into the shopping mall.

　　Now -- (Laughter) -- all of you have done this. I mean, you know, Ben and Jerry's doesn't have liver-and-onion ice cream, and it's not because they whipped some up, tried it and went, "Yuck." It's because, without leaving your armchair, you can simulate that flavor and say "yuck" before you make it.

　　Let's see how your experience

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