Know chalk from cheese informal verb phrase
To be able to distinguish between two individuals or objects (especially by recognizing their differences).
A: I quiz you. Which twin is Nam? B: I can know chalk from cheese. Nam has a birthmark on his neck, and Jack doesn't.
To have no influence on someone, either positive or negative.
Completely dark, without any light or color
I'm scared of the basement! It's as black as a skillet!