This chapter considers the contexts in which children can explore materials as they change shape and state, and observe materials over time. Exploring similarities and differences between materials, observing, explaining and talking about change are all detailed in the 'Understanding the world' Early Learning Goal. Risk assessment prior to the session should also take account of allergies and consider what instructions should be given to the children, for example, what they can touch or eat. The practitioner can support the children's observation by drawing their attention to the salient features of a situation. The SPACE project probed children's understanding of evaporation by asking them to consider the change in water level in a large container over a five-week period. Prediction and explanation are also key skills that can be developed when examining changing materials.