We investigated young children's awareness of the context-relative rule structure of simple games. Two contexts were established in the form of spatial locations. Familiar objects were used in their conventional way at location 1, but acquired specific functions in a rule game at location 2. A third party then performed the conventional act at either of the two locations, constituting a mistake at location 2 (experimental condition), but appropriate at location 1 (control condition). Three-year-olds (but not 2-year-olds) systematically distinguished the two conditions, spontaneously intervening with normative protest against the third party act in the experimental, but not in the control condition. Young children thus understand context-specific rules even when the context marking is non-linguistic. These results are discussed in the broader context of the development of social cognition and cultural learning.