Students usually learn about the states of matter in the upper grades of elementary school. On the basis of this knowledge in physics the particulate theory of matter is introduced in junior high school. The objective of this research was to study the preliminary knowledge students possess regarding the concepts ‘solid’ and ‘liquid’ from kindergarten age (5 year olds) until seventh grade (12 year olds). Our findings show that a child can successfully classify liquids from an early age. This success is due to the idea that ‘all liquids are made of water’. The classification of solids follows a different pattern: Whereas rigid solids are classified correctly by children of all ages, non‐rigid solids are correctly classified by only 50% of children of all ages; the other 50% refer to non‐rigid solids as a separate group. Powders are misclassified by all ages and are referred to either as liquids or as a separate group. Educational implications are discussed.