A snowman, glass of water and steam might look very different but they are made of the same stuff! Just like any substance, water can exist in three different forms, called states: solid, liquid and gas. The state will change when the substance is heated.
As a solid, a substance has a fixed volume and shape and is usually unable to flow, except in the case of glaciers. For instance, an ice cube or snowflake is the solid state of water.
When a solid is heated, it turns into a liquid. As a liquid, a substance has a fixed volume, but its shape changes to fill the shape of its container. For instance, a glass of water is the liquid state of water.
When a liquid is heated, it turns into a gas. As a gas, a substance does not have a fixed volume or shape. Gas expands to fill the shape and volume of its container. For instance, the steam that comes out of a hot teakettle, making the whistle sing, is water in the form of a gas.
Heat causes substances to change their state because, when heated, the molecules within the substance to move around faster. The faster the molecules bounce about, the weaker they are held together.