如果没有穿过之前，中子就掉落到底板，那么它们会反弹到顶板，并被吸收——这样就不会在发射另一端检测到。
If they hit the bottom surface before passing through, they are reflected off and absorbed at the top - and thus are not detected at the other end of the plates.
如果你再这样做一次，可以看到该点来到顶部，又回到底部，然后又开始新的一轮运动。
So, if we do it once more, you see the point now is at the top, and now it's at the bottom. And then we start again.
唯一不同就是当你画渐变时，要从顶部拖到底部，而不是从底部拖到顶部。
Instead of dragging from bottom to top with the? Gradient Tool, drag from the top to the bottom.
他们的方法是按下上升键，让电梯升到顶层，然后一路坐到底层。
Their solution is to press the up-button, ride to the top, and then go down all the way down to the ground.
他们的方法是按下上升键，让电梯升到顶层，然后一路坐到底层。
Their solution is to press the up-button, ride to the top, and then go down all the way down to the ground.
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