when pigs fly

People say "When pigs fly" to describe the impossible.

It seemed for a while that California’s controversial pork law would take effect only when pigs fly.

"Lukas kept saying that he was going to come to Poland, but always chose to play elsewhere, and it gradually became a joke. 'When Podolski signs for Gornik' meant something like 'when pigs fly' in Silesia - that was never going to happen," Wirtualna Polska journalist Piotr Kozminski told BBC Sport.

“For me this bill has a when pigs fly sort of quality to it,” said Democratic Rep. Omari Hardy.

But when pigs fly — that's something unheard of, even by Taiwan's standards.