We are surrounded by objects and substances. We recognise objects that are to serve a specific purpose by their shapes. Similar objects may consist of different materials or substances. Substances, however, are independent of shapes and possess very specific properties. We are able to perceive many of these substances with our senses. For example, we can see, touch or smell them so as to be able to recognise them. Chemists are particularly interested in those substance characteristics that can be measured. On the basis of these measurable properties they can distinguish between substances, identify a specific substance or test it for special use. Models help us to understand phenomena. They depict only specific elements of our reality, thus presenting the world in a simplified way. The spherical particle model, for example, helps us to understand how a scent spreads all over the room or substances disperse in water.