know a hawk from a handsaw
US /noʊ ə hɔk frəm ə ˈhændˌsɔ/
UK /noʊ ə hɔk frəm ə ˈhændˌsɔ/
1.
to be able to distinguish between two very different things; to have common sense
:
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He may seem a bit eccentric, but he certainly knows a hawk from a handsaw.
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Even a child should know a hawk from a handsaw when it comes to basic safety.