According to the Institute of Russian Language of the Russian Academy of Sciences, an optional acute accent (знак ударения) may, and sometimes should, be used to mark stress. For example, it is used to distinguish between otherwise identical words, especially when context does not make it obvious: замо́к/за́мок (lock/castle), сто́ящий/стоя́щий (worthwhile/standing), чудно́/чу́дно (this is odd/this is marvelous), молоде́ц/мо́лодец (attaboy/fine young man), узна́ю/узнаю́ (I shall learn it/I recognize it), отреза́ть/отре́зать (to be cutting/to have cut); to indicate the proper pronunciation of uncommon words, especially personal and family names (афе́ра, гу́ру, Гарси́я, Оле́ша, Фе́рми), and to show which is the stressed word in a sentence (Ты́ съел печенье?/Ты съе́л печенье?/Ты съел пече́нье? – Was it you who ate the cookie?/Did you eat the cookie?/Was it the cookie that you ate?). Stress marks are mandatory in lexical dictionaries and books for children or Russian learners.