Yiddish: If a cat laid eggs, it would be a hen.

It's pointless to speculate about counterfactual situations.
SOURCE: Bernstein, Jüdische Sprichwörter und Redensarten.

די קאַץ זאָל לײגן אײער, װאָלט זי געװען אַ הון.

Di kats zol leygn eyer, volt zi geven a hun.

If a cat laid eggs, it would be a hen.