you're afraid that they'll steal your computer.

你会害怕他们下一次偷你的计算机
Bad thing, nonnegligible chance,

坏事情不可忽视的可能性
and lack of certainty.

以及不确定性
On the other hand, suppose what happens is,

但是如果事情是这样的
this is one of those thieves like you always have in the movies,

就像你在电影里经常看到的一样
where he's such a master thief, or she's such a master thief,

他是个神偷或者她是个神偷
that they take pride in their work,

他们为自己的工作感到自豪
and so they announce it.

所以他们提前预告
They take out an ad in the Yale Daily News and they say,

他们在耶鲁日报上刊登声明他们说
"On Wednesday, April 27th, I shall steal

四月27日星期三我会去偷
the computer from so-and-so's room."

某某房间里的计算机
And it doesn't matter what precautions you take,

然后不管你做了多少预防准备