There are an indefinite number of logically possible hypotheses about the meaning of a word. Since children broaden their vocabulary so rapidly, they must be constrained to give priority to some hypotheses. ‘Mutual exclusivity’ (children assume that an object has only one label) was proposed as one of such constraints by Markman (1987). If children have such a constraint, a novel label for a familiar obje ct should be rejected, even when the context shows it clear that the label refers to the familiar object. Study 1 and 2 tested this hypothesis with 3-year-olds and confirmed it. Study 3 and 4 showed that children, as they grew up, came to take more of the context where the label was used than mutual exclusivity, and as a result of such change in strategy to interpret novel words, 5-year-olds came to accept the second label for a familiar object when it was presented in an appropriate context.