1. 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)

1. I tried to give Grandma my new recipe for spaghetti sauce, but she refused. You can’t teach an old dog new tricks.

2. 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.

3. 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.