During the two-and-a-half hour moon walk, the astronauts frolicked in the weightlessness of outer space and planted an American flag in the lunar surface.

VOA: standard.2009.07.16

And shortly thereafter this thickening of the outer surface is dimpling in, its invaginating.

增厚的外胚层很快就,向内凹陷。

斯坦福公开课 - 7个颠覆你思想的演讲课程节选

And shortly thereafter this thickening of the outer surface is dimpling in, its invaginating.

增厚的外胚层很快就,向内凹陷。

斯坦福公开课 - 7个颠覆你思想的演讲课程节选