if you've got a hill or valley and there's a cart on wheels it's going to go down to the bottom.

如果我们有一座山或者山谷，并且,有一个有轮子的车，这辆车会自动滑到底部。

麻省理工公开课 - 热力学与动力学课程节选

Now, the amazing thing is, if you go to Mexico you can go to museums that have children's toys from pre-Columbian Mexico with wheels.

现在神奇的事情是，如果你去墨西哥,你可以去博物馆,那里有前哥伦比亚墨西哥时代的,带着轮子的幼儿玩具。

耶鲁公开课 - 金融市场课程节选