Piagetian tasks have more to do with the child's ability to inhibit interference than they do with the ability to grasp their underlying logic. Here we used a chronometric paradigm with 11-year-olds, who succeed in Piaget's conservation-of-weight task, to test the role of cognitive inhibition in a priming version of this classical task. The experimental design was such that the misleading strategy "number-equals-weight" to inhibit on the prime (a Piaget-like item with weight/number interference) became a congruent strategy to activate on the probe (a subsequent item where weight and number covaried). A negative priming effect of 142 ms was observed for the prime-probe sequence. This result is consistent with the prediction that success on Piaget-like tasks (the prime) requires an inhibition process.