Pigs might fly.
British,American & Australian informal
Pigs can fly.American informal
something that you say which means you think there is no chance at all of something happening
  \x05'I'll pay you back on Friday,I promise.' 'Yeah,and pigs might fly.'
(has the) cat got your tongue?
SPOKEN said when someone will not answer you or talk to you ◆ OFTEN USED BY ADULTS TALKING TO CHILDREN:
"Did she like it?" "Dunno." "Did you speak to her?" "Dunno." "Cat got your tongue?" "No."