There are three hallmarks of core linguistic properties. First, they are expected to be manifested in typologically different languages. Second, they should unify superficially unrelated linguistic phenomena. Third, they are expected to emerge early in the course of language development, all things being equal (Crain, 1991). The present study investigates a candidate for a core linguistic property, namely the semantic property of downward entailment. We report the findings of two experimental studies of children's knowledge of downward entailment. These experiments explore two different aspects of downward entailment, in a study with Mandarin-speaking children. Taken together with previous research findings, the results of the present study support the conclusion that downward entailment is a core property of human languages.