One of my old chestnuts but I don't care.

Little boy goes into the frozen food shop. He says to the man, "My mum says can I have a packet of Birdseyes pissoles please, mister."

The man frowns, then smiles. "Ahah, little boy, I think I know what you want, but it's an R, not a P!"

"OK," says the little boy, "can I have a packet of Birdseyes arseholes then please, mister."