Recent research suggests that young children assume, as a default, that a word applied ostensively to an unfamiliar object (e.g. a man riding in a car) is a basic-level count noun (e.g. person) rather than a situation-restricted count noun (e.g. passenger). In this experiment, 14 mothers and their children (mean age 3;7; range 3;0 to 4;6) participated in a story-book reading session in which the mothers taught their children both a basic-level count noun and a situation-restricted count noun for a series of object drawings. An analysis of mothers' spontaneous teaching strategies revealed that, for a given object, they typically taught a basic-level count noun before a situation-restricted count noun. Furthermore, they tended to teach basic-level count nouns exclusively through estensive definition, especially when the objects were unfamiliar. Moreover, they were (1) more likely to use estensive definitions and (2) less likely to provide additional information concerning the application of the word, when teaching basic-level count nouns than when teaching situation-restricted count nouns. The results provide new evidence that word teachers, like young word learners, assume that a basic-level count noun is the psychologically-privileged candidate for an ostensive definition involving an (unfamiliar) object.