pigs and cows don't fly

a phrase used to mean "normal and civilized (however one defines these words) people don't behave the way you do.

a boy, after having been to physical science class in elementary school and having learnt the basics of what electricity is, comes home and tries putting scissors in the kitchen plug.

His mother starts screaming at him:

mother: is this f-sag you twit? what are you doing, you'll create sparks and burn the damn house down. Pigs and cows don't fly. Go to your room now. She then takes a broom and starts chasing him around the house.

son: I can attach balloons to anything i want, (turn anything I want into a fad or trend) no?