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Phrase(s): You cannot teach an old dog new tricks.
Prov. Someone who is used to doing things a certain way cannot change. (Usually not polite to say about the person you are talking to; you can say it about yourself or about a third person.) • I’ve been away from school for fifteen years; I can’t go back to college now. You can’t teach an old dog new tricks. • Kevin’s doctor told him not to eat starchy food anymore, but Kevin still has potatoes with every meal. I guess you can’t teach an old dog new tricks.
McGrawhill's American Idioms And Phrasal Verbs