وَاضْرِبْ لَهُم مَّثَلَ الْحَيَاةِ الدُّنْيَا كَمَاءٍ أَنزَلْنَاهُ مِنَ السَّمَاءِ فَاخْتَلَطَ بِهِ نَبَاتُ الْأَرْضِ فَأَصْبَحَ هَشِيمًا تَذْرُوهُ الرِّيَاحُ ۗ وَكَانَ اللَّهُ عَلَىٰ كُلِّ شَيْءٍ مُّقْتَدِرًا
And present to them the example of the life of this world, [its being] like rain which We send down from the sky, and the vegetation of the earth mingles with it and [then] it becomes dry remnants, scattered by the winds. And Allah is ever, over all things, Perfect in Ability. [18:45 ; http://quran.com/18]
Imam Qurtubi explains:
- Water does not remain in one state (gas, liquid, solid) and hence changes. This world is not constant and is changing.
- Water does not last and disappears (or evaporates). The same happens to this world.
- No one can enter water without getting wet. No one enters the world without being saved from trials and tribulations.
- In limitied quantities water is beneficial and allows plants to grow; excessive quantities destroy it. Taking from this world in limited amounts is beneficial while excess is harmful and ultimatley destructive.