Chalk and Cheese
I know the difference between chalk and cheese. Between what is worthless and what is valuable, between a counterfeit and a real article. Of course, the resemblance of chalk to cheese has something to do with the saying, and the alliteration helps to popularise it.
“This Scotch scarecrow was no more to be compared to him than chalk
was to cheese” —
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894