如果你想自己搞清答案就在这里暂停视频！
Pause the video now if you want to figure it out for yourself!
答案即将公布：3答案即将公布：2答案即将公布：1
Answer in: 3 Answer in: 2 Answer in: 1
你可能觉得你需要拿走全部4个箱子以检查另外一面的内容。
It may seem like you need to inspect all four boxes to see what’s on the other side of each.
但其实，你只需要看两个箱子。
But in fact, only two of them matter.
让我们回顾一下规定。
Let’s look at the protocol again.
它只说了偶数箱子必须要有红色封顶。
All it says is that even-numbered boxes must have a red top.
它并没有对奇数箱子做出任何规定，所以我们可以忽略标着7的那个箱子。
It doesn’t say anything about odd-numbered boxes, so we can just ignore the box marked with a seven.
那么，那个红箱子呢？
What about the box with a red top?
我们难道不需要检查它的底部是偶数吗？
Don’t we need to check that the number on the bottom is even?
事实证明，确实不需要。
As it turns out, we don’t.
规定说的是如果一个箱子标着偶数，那么它的封顶应该为红色。
The protocol says that if a box has an even number, then it should have a red top.
它并没有规定只有偶数箱子可以有红色封顶，或者一个红色封顶的箱子必须有偶数标记。
It doesn’t say that only boxes with even numbers can have red tops, or that a box with a red top must have an even number.
规定只要求了单方向的。
The requirement only goes in one direction.
所以我们不需要检查红色封顶的那个箱子。
So we don’t need to check the box with the red lid.
不过我们确实需要检查黑色封顶的箱子来确保没有不小心把黑色封顶放到了偶数箱子上。
We do, however, need to check the one with the black lid, to make sure it wasn’t incorrectly placed on an even-numbered box.
如果这还是有点难理解，让我们看一个不太一样的问题。
If that still seems hard to wrap your head around, let’s look at a slightly different problem.
想象一些装着杂货的袋子。
Imagine the boxes contain groceries.
你看到一个标着运往牛排店还有一个运往素食餐厅。
You see one marked for shipment to a steakhouse and one to a vegetarian restaurant.
然后你看到另外两个反着放的箱子：一个标着含有肉，另一个标着含有洋葱。
Then you see two more boxes turned upside down: one labeled as containing meat, and another as containing onions.
你需要检查哪个呢？
Which ones do you need to check?
这很容易——确保肉不会被运到素食餐厅，以及运到素食餐厅的箱子里没有肉。
Well, it’s easy – make sure the meat isn’t being shipped to the vegetarian restaurant, and that the box going there doesn’t contain meat.
洋葱去哪里都可以，而运往牛排店的箱子可以含有任意一种食材。
The onions can go to either place, and the box bound for the steakhouse can contain either product.
为什么这个情况感觉更容易？
Why does this scenario seem easier?
本质上，它是同一个问题——箱子上面有两种可能底下也有两种。
Formally, it’s the same problem – two possible conditions for the top of the box, and two for the bottom.