When we look out at the world around us, we see landscapes that have been shaped and sculptured largely by the forces of water. For over thousands of years, water has carved out riverbeds, worn down mountains, and formed the patterns of erosion and sediment that make our landscapes so unique and varied. But to truly understand how water can transform something seemingly permanent as the planet itself, we need to first understand what water is and how it behaves in different environments.

Water appears in three distinct forms: solid ice with its crystalline structure, vapour which takes the form of moisture and air and soil, and liquid water found in oceans, rivers and groundwater under our feet. In nature, water cycles through its different states. Liquid water evapourates into vapour in the air before condensing back into liquid and falling to the ground as rain.

Eventually, all of this water can freeze into solid ice. When rain falls, it gathers in tiny droplets that flow across the ground, forming small rivulets, streams and eventually large rivers. Over time, this flowing water slowly but persistently erodes our planet's surface because moving water molecules are incredibly strong. If we look at what happens when water freezes into ice, we'll see a unique property of the water molecule that sets it apart from other crystals.

If I drop a stone into the water... it sinks. However, if I put these ice cubes into the water, we see something different. It floats. And ice will always float regardless of if it's these cubes or the largest icebergs around the North Pole. This is because liquid water reaches its highest density at four degrees Celsius, while the water molecules that are bound in ice always have a lower density than liquid water.

If ice were to sink like other solids, ice would accumulate at the bottom, leading to a solid mass of ice forming over time. If ice had behaved like this, the oceans and lakes would eventually freeze from the bottom up and make our planet entirely uninhabitable. Now we have looked at some of the unique properties of the water molecule and why water not only shapes our landscapes but also sustains life in all its forms. In the next module, we'll explore why ice is one of nature's strongest forces.