We commonly attribute an understanding of language to others including very young infants, and, more controversially, to other animals and computers. Although we adults attribute or "ascribe" understanding to very young children, only in the late preschool years do the children themselves begin to ascribe understanding to themselves and others a competence that comes with learning the meaning of the word "understand." It is argued that ascription of understanding to others allows the creation of shared belief while self-ascription allows one to introspect on one's understanding: to know that one understands, to understand expressions that young children would simply reject as false, and to understand hypotheticals and counterfactuals. This competence applies to both understanding spoken expressions and reading comprehension.