The addition of the phrase بقدر (In due measure) shows how feeble and utterly powerless man is, because if the very things without which no life can exist exceed a certain limit, they become the cause of death and destruction. Water is a case in point without which no living thing - man or beast - can survive. Torrential rains coming down from the sky cause a deluge which becomes a source of misery for human beings. But Allah is kind and merciful and He sends rains which, while fulfilling man's needs, do not turn into flood except at those places where it is His will that they should be engulfed in water. Another point to consider is that if man was to obtain his daily requirement of water through daily rainfall, that too would cause extreme discomfort and suffering, because he is not by nature adapted to live in an environment where rain falls every day. Moreover the conduct of business would become impossible in such conditions. On the other hand, if the total quantity of water that men would need for a whole year, or for six months or even for three months were to come down in a single downpour and people are told to store their quota of water and use it according to their daily needs, that would be an impossible task because, apart from lack of adequate storage capacity, the water would become foul and unfit for consumption. Therefore Allah has regulated the supply of water in such a way that when rains come, the water saturates the earth and the plants and then it flows into lakes, ponds and natural depressions where it is used by men and animals according to their needs. But the water stored in this manner can only last for a limited time and men of different regions need a continuous supply of fresh water. To meet this eventuality Allah has designed a system by which very large quantities of water are converted into oceans of ice and snow and placed on top of mountains where the atmosphere' is pure and free from pollution. Slowly and gradually the snow melts and the water seeps through the pores in the mountains and flows into the network of underground water channels which carry it to all parts as springs and flow on the surface of the earth as streams, water courses and rivers. This fresh running water provides drinking water to men and animals and irrigates fields which yield food and fodder for all living things. Another part of the water percolates deep into the ground and becomes sub-soil water and is drawn up from wells. This entire design and process is covered in the Qur'an by a simple and short sentence فَأَسْكَنَّاهُ فِي الْأَرْ‌ضِ (23:18). A point is also made at the end of the verse that the sub-soil water which is drawn from wells has been placed by merciful Allah at a depth from where it can be drawn easily because by its nature, water flows downwards and could have gone down to a depth from where it would have been impossible to draw it out. This is explained by the sentence وَإِنَّا عَلَىٰ ذَهَابٍ بِهِ لَقَادِرُ‌ونَ and, of course, We are able to take it away. (23:18).