Two lrishmen are hammering floorboards down in a house.
Paddy picks up a nail, realises it's upside down & throws it away.
He carries on doing this until Murphy says, "Why are you throwing them away?"
"Because they're upside down," says Paddy.
"You daft prat," replies Murphy, "save 'em for the ceiling!!"
Thanks Mom for sharing this with me - it gave me a smile at the end of a long day of renovations.