During early childhood, children interact with objects around them. During these interactions, they develop their own concepts about matter, even before encountering the word matter in school. When they encounter the concept of matter in school, they develop a beginning notion of matter as the "stuff" that makes up the things around them. This column focuses on promoting learning through assessment. This month's probe can be used to elicit young children's ideas about matter. The probe reveals commonly held ideas elementary students have about the types of objects, materials, and substances that are considered matter.