In prior grades students learned about liquids and solids. In grades 2-3 students learn to identify different physical properties of materials (matter) and to realize that an object may be made from several different types of materials. They also learn that properties of materials change when environmental conditions change. Water, for example, changes to a solid when the temperature drops below 0°Celsius. Although few students at this age will fully understand that water may change to an invisible gas (e.g., water vapor) when left in an open container overnight, they can start to become familiar with changes of state by observing ice cubes freeze and then melt, and seeing water turn to steam when heated. Looking closely at matter to describe its characteristics will eventually lead to understanding the basic nature of matter and its physical and chemical properties.