With peaks and valleys, folds and rifts, there are plenty of hiding places for a virus to stick.

手上高低交错的褶皱裂纹中，有许多能够让病毒附着的地方。

Unfold that sheet, and there's a pattern of lines, each of which represents a concave valley fold or a convex mountain fold.

展开那张纸，有一种线条图案，每条线代表一个凹谷折或凸山折。

Second, the number of mountain and valley folds at any interior vertex must differ by exactly two— like the three valley folds and one mountain fold that meet here.

其次，任何内部顶点处的山折和谷折的数量必须恰好相差二——就像在这里相交的三个谷折和一个山折一样。