GRE阅读文章的第三种常见结构是“现象解释型”。文章开始提出一个现象，然后解释现象。解释是主题。有多个解释时，作者一般对前面的解释持负面评价，对最后的解释持正面评价。提出现象时的标志词有：phenomenon, fact; problem, difficulty, puzzle, question等。例如，

What causes a helix in nature to appear with either a dextral ("right-handed,"or clockwise) twist or a sinistral ("left-handed," or counterclockwise') twist is one of the most intriguing puzzles in the science of form…What mechanisms, control handedness and keep left-handedness rare?

It would seem unlikely that evolution should discriminate against sinistral snails if sinistral and dextral snails are exact mirror images, for any disadvantage that a sinistral twist in itself could confer on its possessor is almost inconceivable. But left- and right-handed snails are not actually true mirror images of one another…

But this evolutionary mechanism combining dissymmetry, anatomy, and chance does not provide an adequate explanation of why right-handedness should have become predominant…

Here, the evolutionary theory must defer to a theory based on an explicit developmental mechanism that can favor either right- or left-handedness…Thus, the path to a solution to the puzzle of handedness in all snails appears to be as twisted as the helix itself.

是什么样的原因致使自然界中的螺旋体呈现出右旋(“右向旋转的”，或顺时针的)或左旋(“左向旋转的，”或反时针的)？这是形态科学中一个最引人入胜的不解之谜......究竟是一些什么样的机制控制着旋转方向，并使左旋的比例为数稀少呢?

假如左旋和右旋蜗牛呈完全一致的镜像对称的话，那么进化对左旋蜗牛不利，这近乎不太可能。几乎无法想象左旋本身会给左旋者带来任何不利之处。然而，左旋与右旋蜗牛彼此间实际上并不呈真正的镜像对称......

但是，这种结合不对称、解剖学和偶然性的进化论机制并未提供一种充分的解释，以说明蜗牛的右旋何以会成为主要的旋向......

于是，进化论必须让位于支持右旋或左旋的明确的发育机制为基础的理论......因此，解决所有蜗牛身上旋向之谜的道路似乎与这一螺旋体本身一样曲折复杂。

上文在第一段最后提出一个问题：“究竟是一些什么样的机制控制着旋转方向，并使左旋的比例为数稀少呢?ァ闭飧鑫侍饩褪且桓鱿窒螅郝菪体中左旋的比例少。有问题就有解答，有现象就有解释。第二段从进化论的角度解释这一现象。但是，GRE阅读文章中一般对进化论的解释持负面评价，所以需要寻找新的解释。第三段否定了进化论的解释。在末段，作者以发育机制（developmental mechanism）解释“为何左旋少、右旋多”。