Water is the essential substance for life on Earth. The natural patrimony of all living beings and the material and cultural heritage, a common good, a shared resource, a treasure trove of humanity. The nature of water, water at rest and in motion, the water cycle and its management are addressed here in a multidimensional, comprehensive and transdisciplinary approach. The book focuses on water without complying with any disciplinary approach: philosophical, historical, physical and mathematical issues are merged with traditional hydrochemistry, hydrostatics, hydrodynamics and hydrology to provide an overview of basic knowledge on Earth's water. These five lectures can be delivered to any undergraduate STEM student to introduce them a valuable, basic, multidisciplinary knowledge on water. Non-specialists will find here an insight of water made as simple as possible, but not simpler than required.