Unfortunately, that critical size is 150 million molecules.

不幸的是，这个临界体积是 1.5 亿个分子。

If not caught, a fatigue crack will eventually reach a critical size where it can propagate rapidly, leading to a fracture.

如果不及时发现， 疲劳裂纹最终将达到临界尺寸， 并能迅速扩展，导致断裂。

Fortunately, they don't have to — they can start off at that critical size by condensing onto one of the gajillions of little pieces of dirt floating in our atmosphere, and then grow and grow until they're a droplet in a raincloud.

幸运的是，它们无需孤军奋战，它们可以将附着到大气中的尘埃上作为最终凝聚到临界体积的起点，不断凝聚，直到变成雨云中的一粒小水滴。