teach your grandmother to suck eggs
US /tiːtʃ jʊər ˈɡrændˌmʌðər tə sʌk ɛɡz/
UK /tiːtʃ jʊər ˈɡrændˌmʌðər tə sʌk ɛɡz/
1.
to try to teach someone something they already know, especially someone who is more experienced than you
:
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I tried to explain how to use the new software to my boss, but it was like teaching my grandmother to suck eggs; she's been using similar programs for decades.
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He started giving me advice on cooking, but I've been a chef for twenty years. It was definitely a case of teaching your grandmother to suck eggs.