Water
Water is very important for biological,there is no water,no life on Earth.All the animals and plants need water.Human beings also need water.
Water is almost everywhere.Even in the world's driest place,the air also have water.
Water,as you know,there are solid,liquid and gaseous.When it is solid,it can be as hard as brick; When it is liquid,you can use it poured from the container; When it is gaseous,you can not see or feel it.
While 70% of the Earth's surface is water covered the world,there are still many parts of the water.Therefore,we have to make good use of the water on the planet.