But rivers don't have umbilical cords, so why do they have innies and outies?

但是河流又没有脐带，那它们为什么会有凹凸的区别呢？

But in general, belly buttons have two basic different shapes: 'innies' and 'outies.'

但一般来说，肚脐有两种基本不同的形状：凹脐和凸脐。

And innies are ripe for colonization, not only by lint, hair, and dead skin cells, but also by bacteria.

小狗身上很容易滋生细菌，不仅是棉绒、毛发和死皮细胞，还有细菌。

Rivers also have innies and outies, not belly buttons, but mouths: where rivers flow into the sea, the land either pokes out or bends inward.

河流同样也有凹凸之分，不过不是肚脐，而是河口：在河流入海的地方陆地会向外延伸，或者向内弯曲。