Totalement metaphorique, n'est-ce pas?
"quand les poules auront des dents" se dit en Anglais "quand les cochons voleront"
I'm learning English, and found out that I should put phrases and expressions in a safe place to be sure to remember them when needed. I began by this one :"when pigs will fly" because it's so cute . In French you would say "when hens have teeth" wich is not as much fun.
Many thanks to Sam for helping me on the spelling and inspiration ( and for being so patient with us French people and our bad, bad, very bad English).
There is also a phrase in French that says "like giving jam to pigs", but the way we say it can be also translated by "pigs jam". Et voila!