A fly can walk up a wall. It can walk upside down.

苍蝇能够在墙上行走。它也可以头朝下走。

youdao

I trained her a while each day. After 30 days she could agilely walk upside down.

我每天训练她，30天之后，它已经可以灵活地倒著走路。

youdao

Ok, if we look at it near here, if we walk along this way, the surface is to our right.so, we should actually be flipping things upside down. The normal vector should be going down.

我们看看这里，如果我们沿着这条路走，曲面就在我们的右边，我们应该是把它颠倒一下，这个法向量也应该向下。

youdao