Water can exist in three forms: solid (ice), liquid (water), and gas (water vapor or steam). Temperature determines which form water takes. Below freezing (0 degrees Celsius), water becomes solid ice. Above freezing, it is liquid water. When heated enough, liquid water becomes water vapor, an invisible gas. Water changes between these forms all the time in nature -- ice melts into water, water evaporates into vapor, and vapor condenses back into liquid.
Provide ice cubes and let children observe them melting at room temperature. Boil water (supervised) and show steam rising. Hold a cold surface (like a cold metal spoon) above the steam to show condensation. Freeze water in a cup to show liquid becoming solid. Label all three forms and discuss where children see each one in daily life (ice in a drink, liquid in a faucet, steam from a pot).
Water is one of the most amazing substances on Earth because it can be three completely different things depending on the temperature. Put water in a freezer and it becomes ice -- hard, solid, cold. Leave ice on a counter and it melts back into the familiar liquid water you drink and swim in. Heat water on a stove and it turns into water vapor -- an invisible gas that floats away into the air. Same substance, three different forms.
The key that controls which form water takes is temperature. Below 0 degrees Celsius (32 degrees Fahrenheit), water freezes into ice. This is why puddles freeze in winter and why ice cubes stay solid in the freezer. Above 0 degrees, ice melts into liquid water. And when water gets hot enough -- at 100 degrees Celsius (212 degrees Fahrenheit), it boils and rapidly turns into water vapor. But water does not have to boil to become a gas. Even at room temperature, some water molecules at the surface escape into the air. That is why a wet towel dries and a puddle on the sidewalk slowly disappears.
Here is something important: these three forms -- solid, liquid, and gas -- are not different substances. They are all water. An ice cube is water. A glass of drinking water is water. The invisible vapor rising from a hot bath is water. The molecule is exactly the same (scientists call it H2O). What changes is how the molecules behave. In ice, the molecules are locked in a rigid pattern and cannot move around. In liquid water, the molecules slide past each other freely. In water vapor, the molecules fly around independently with lots of space between them.
You see water changing forms every day. Frost on a window is water vapor from the air that froze on the cold glass. A cold glass of lemonade on a hot day gets droplets on the outside because water vapor in the warm air condenses into liquid when it touches the cold glass. Snow melts on your warm hand. Water boils on the stove. Every one of these is water moving between its three forms -- and these changes are what drive the water cycle that brings rain, snow, and weather to the whole planet.