Demonstrating is a traditional method in teaching science that can raise interest and encourage pupils to think about a topic. While demonstrating, the teacher can focus the pupils' attention on the relevant facts and introduce scientific principles and concepts. Through discussion and actively making observations and inferences, rather than looking passively at what is happening in the demonstration, the pupils gain experience of the scientific way of thinking (Ogborn et al., 1996). This may mean that the lesson deviates from the plan and a certain amount of improvisation is required by the teacher. Herein the authors provide some ideas, particularly relevant for new teachers, about making demonstrations active rather than passive experiences. Disciplined improvisation is discussed and an example of demonstration and improvisation using a 4th grade (10-year-olds) science lesson is provided.